Hi /u/adi_shakti. I'm going to tell a story. Bear in mind that it's a story, and fictional at that. So I'm not "lying" to you in any of this, but there are always other perspectives. You can view it in two ways. Here is a "general conceptual holography" of it. See what you want.

With Kant, there are three prominent critiques. I'll begin with Pure Reason. It's only fitting. In it, he starts by a few preliminary observations to "reason out" (so to speak) what this peculiar aesthetic must have been about. It's called transcendence, or per Kant it is the "transcendental aesthetic". It is a kind of "skin" or "transparent overlay", I guess, for a expanded, albeit rigorous system of conceptual operations. It's just a concept, that we dwell in. Negatively, if you want to observe this, Kant lies down a mesh-like field of sorts, which is ever "before" the mind. And this invisible field of ideas contains what, exactly? The categories of/for thought, which he puts in a great effort to identify, each in turn.

He then proceeds, from this spaciousness, to further define his kind of aesthetic, throughout Pure Reason. He goes on to specify its "range of applicability", in Practical Reason. He carries through to understand its "motivations" and "motive power", if you will, in Pure Judgment. So this means that you're presumably quite familiar with that, already. But why is any of this Kantian enlightenment stuff (mildly) important, in the first place? We'll see what happens. For now, it is sufficient to show that Kant was building an argument for it, as each one - in series - entered in to supplant all of the others. You picked up on the last third of it, but all things considered it's a huge project.

Look for yourself: this is colossal, if you think about it. All three, in working together, may begin to productively advance forward, in a certain way. Or that was the main idea, at the time, as "guided" by Pure Reason. That's because, in starting with Pure Reason, it is as though it were necessarily the case (i.e. argumentatively) what Kant was getting at. I say "as though" necessarily because, later on, with Hegel, there will be some push-back to the whole idea of categorization, which is indeed (I suppose) further mitigated by the likes of Kierkegaard et al. According to most continentals, this advancement was fundamentally mistaken for some narrative of "historical progress".

Backtracking, Kant's grand project was supposed to happen out of a certain necessity: Pure Reason itself. Kant was submitting something to be the case, over and above merely Humean impressions of things. And yet, by Nietzsche, one might say, this framework had been "exploded" entirely, into many bits and pieces of fragments. It's shrapnel. So, it doesn't mean a thing, even if technically speaking, it is true enough: Kant was right. Then, an assortment of people variously tried to put it ("critical theory") all back together, in the 20th century, with the existentialists and whatnot. We're postmodern now, so more on the postmodernists in a bit.

As for the argument itself, it's from the 1840's, which is after Hegel died. That's when it was actually "rendered thematic", "immanently realized", "concretized", etc. It took a little while to get there, i.e. people got caught up in fanciful songs and Romantic poetics in-between, but Kant's project finally "settled in" at around that time. Because foundation. It instantiated a "hard correlation" between all things in this underpinning work of negative enframing, which grew considerably "weaker" in the late 19th - early 20th century, until nothing but an ideology of "correlation" remained in the late 20th century, as a leftover or residue of Kant's thought. What happened??

Now, there happened a string of philosophers who are "speculating" (really!) that, at some point in Kant's work, he made a "mistake". They may not exactly call it that, and more obstinately they are, as you rightly say, "...denying the reality of this transcendental". As a consequence, the speculative realists are drawing out the implications of the "mistake" as-if Kant's grand project never really happened, in effect continuing the general trajectory of Hume and the Humeans. I can only offer impressions. To keep it brief, broadly speaking, they attempt to justify this move by the pure contingency of it. That doesn't obviate "factuality" in actual practice, does it?

Although, it might be worthwhile here to notice how Kant's object is quite the undertaking. It is important to bear in mind that Kant was, at the time, more or less undertaking the whole of the "philosophical canon" all at once. I mean this quite literally: a taking-under. That is why it is sometimes said to be about a type of "negative laying-out" of the categories of/for thought. With respect to all of the philosophers, Kant is (apparently) understanding them, learning from them. Oddly enough, he is "making sense" of the philosophers by, well, keeping 'em in their place. Hence, we have his famous remark about Hume and being "awakened" from some dogmatic slumber or another. So, who really knows what is and is not a "mistake"? One must live transcendence, after all.

Meanwhile, all along, there were others who have insisted that "there is no going back" for we are now enlightened. It's like a switch-point, and it suffices to just turn on the lights. There - it's done! Analytic philosophers, even to this day, saw the significance of Kant's work as more or less ongoing, continuous, and constant. It's work, not just some big idea. It's not even simply your imagination, although it becomes your imagination. And, surely enough, it's nevertheless working just as Kant described it in his original Critique of Pure Reason. Cliché? Absolutely. So, for those working in analytic philosophy, the implemented foundation didn't actually break up in practice, since the meaning of it wasn't ever the point. The grand project was recognized as, at the very least, a good one. To wit, there was a value in it that could be "reasonably" discerned: goodness.

That was perhaps enough to keep them working on logical things, feeling as though they were getting somewhere in their experience. Along the way, one could continue to provide "good reasons" for it, even if there were also serious, "real reasons" for actively dismantling it. Nietzsche, again. Whereas, in philosophy departments, many tenured professors have been since Kant discussing the ramifications of the whole "Kant versus Hume" shtick. Scholasticism is alive and well, it seems. They study things and things of things, etc. They became neo-Kantians or neo-Humeans, etc. instead of (or, at times, alongside) the Hegelians or Marxists. They were generally concerned with the task of hermeneutics more than they were with dialectics, not being confident enough in knowing about the action mechanism by which "the dialectic" logically functions except vaguely "in spirit" (whatever that does). They were always unsure about the prospects of "phenomenology", as well.

But the dialecticians (now, our favorite continental theorists) quickly respond to the by posing a simple counterpoint: "what is this ... more than? what then?" And it went back and forth for a bit, in the early 20th century, unto postmodernism. Enter the analytical pragmatists, in the middle of the century. They would say things like almost is good enough, attempting to water down the whole "debate" through an appeal to democratic pluralism. What's the real issue, anyway? What ever happened to forgetting? Why is everything "dark" now? Why say "dark enlightenment"? In returning to Kant to see what the actual situation was to begin with, the speculative realists are now pushing a different argument altogether. The way I do it is different. They are abandoning this one more or less "phenomenological" method that the continentals routinely employ, but it simply isn't working.

At the same time, they are "denying" that it ever happened, at least in the manner that Kant told his side of the story. They are saying, roughly, that "...it could have happened like that, but it didn't". What gives? Meillassoux is for me a bit suspect, owing to this intrinsic contradiction; I like Brassier, though. This matter of factness hasn't been resolved, yet, except "speculatively" so. I would like to see the resolve, although there is no reconciliation. So, your intuition is on point, here, that I guess we are all in a process of learning. But what are we supposed to be learning in this process? Kant would say it is about Pure Reason informing a kind of cosmopolitanism for the pursuit of "perpetual peace".

However, the speculative realists want to use something a bit more extreme in this than to appeal to how there is somehow a "process" in which we are involved: they want to forcibly uphold their (more or less Humean in spirit) version of the Kantian proposal or, well, "counter-argument" to the situation. The whole situation is the issue, not any of the specifics or details within it which are being worked out in process. I'm right, you're wrong. End of story.