To preface, I'm trying to explain

the claim of "Smash 4's aerials are so much more laggy"

to you.

For my initial reply I just picked a few characters across a wide spectrum (standard character, worst case scenario, and super heavy) to disprove the initial general claim of landing lag being drastically worse

You gave 3 characters as an example, one of which got pretty significantly more laggy while the others hardly changed and had lag added to some of their moves as well. That doesn't disprove the argument that Smash 4 aerials have more lag at all. Mind that "much more lag" in that context can also be interpreted quantitavely. As I said, "twice as much lag" was hyperbole, but a looot of aerials were negatively affected. And even if that wasn't the case, I think most agree that less lag on aerials is a good thing so having an aerial nerfed at all in that regard sucks.

(...) it's more a move-by-move basis on what's laggier and what's not instead of a universal trend (...)

Now I had to rummage through frame data after all. I looked at around half the brawl cast and there definitely is a trend towards more lag. Sure it's not that many frames, but maybe this shows what I mean by 'quantitatively'.
It also doesn't help that many of the non-DLC newcomers have pretty bad frame data as well. I#d like to play WFT but her aerials are like, pathetic

Marth is a character who was outright nerfed and made slower all over (...), yet not even he had everything made laggier

That sounds like a fallacy. Marth still got significantly less mobile and it's annoying.

Another reminder that the aerials are nair/fair/bair/uair/dair, so bair is the aerial with decreased landing lag, not uair You seem to have made a mistake, it is uair.

And shield pushback is still very much existent and matters in Smash 4 Disagree.

with Smash 4's faster falling speed, especially fast falling, that's less mandatory time you have to spend in the air, so that will facilitate the speed at which you can attack aerially.

You can't make a general statement like that. For example, this increases the frequency of you having to land between attacks (with more lag on average).

(concerning Zelda) That sounds like a move-specific issue

Yea I said this was an example? This also affects zelda a lot though. In Brawl, you could actually use her side aerials to space. Now her only quick aerials have hitboxes that are like inside of her hurtbox and if you miss a Fair or bair you're just completely screwed. And having to do a fullhop to do things you could do in a shorthop before feels like 'twice as much lag'