You're asking for a new SKU. That in itself is not free. You are asking for something that needs to be engineered, developed, manufactured, and distributed across the globe all for a secondary hard drive. Unless you wanted to charge people an extra hundred or two for this feature, there's no way to justify the cost of such a small feature targeted at a small audience, and for what? A few seconds of reduced load times?
The fact that we can have external drives, and the fact that those drives running through USB3.0 gives us better performance over the stock drive, and the fact that you have the option to use an SSD as an external drive, on top of the fact that the limitation of the read speed of the internal drive is not necessarily tied to the speed of the drive itself means that you are asking for a feature that would cost a lot of money to implement to benefit a very small group of people.
Who is this marketed towards? Microsoft is going to have a hard enough time selling a premium console at a premium price, and a feature like this will push that cost well past the comfort zone of most people. I consider myself an enthusiast, I don't even bother using the internal drive of my Xbox One due to the read speed, and I wouldn't ever consider buying a more expensive Scorpio just for an internal SSD.
We've seen the speed differences between different external drives. Switching to an SSD gives small but somewhat significant gains, that's true, but the difference is the load decreases are measured in a handful of seconds. Most people wouldn't pay extra for this feature as the objective of the idea, to reduce load times, is already provided to us by using optional external HDDs. The benefit of this system is we can choose whatever SSD we want and pay however much we want for it. The idea of creating an additional SKU with a tiny internal SSD is absurd. Who would buy such a thing? I know I wouldn't. I'd rather spend that money on a 1TB external SSD of my choice rather than a preinstalled 128GB internal, even if that's at the cost of a few seconds of load times.
What you're suggesting is a SKU whose only feature is already redundant and pointless and stacked on top of an already expensive product. If you're the kind of person that's willing to spend extra money solely to reduce load speeds by a handful of seconds, you should be building a gaming PC. Consoles are not about that experience, plain and simple.