But at some point making your table larger just defeats the purpose of the library itself. Your table becomes the new library, and you have to walk around on it and look up things in these piles of books. So you make a smaller table in the middle of the big table.
Your fundamental limitation is how small you can make a memory cell, not how big you want to make a cache. That’s akin to making the books smaller print size so you can fit more on the same size table.
The other physical aspect we’re dealing with is propagation delay and physical distance. That’s where the library analogy really shines: if there’s a minimum size to a book and a minimum size of you (the person doing the research) this corresponds roughly to minimum cell sizes and minimum wire pitch, so you’re ultimately limited in the density you can fit within a given volume.
Once you have enough registers, having more mean lowers active utilization for any given instruction (bad use of space, vs. fast pipelined access to cached stack) or higher levels of parallel instruction dispatch (much greater complexity, and even greater inefficiency for branching misses).
Then you have to update instruction sets, which could be impossible given how tightly they fit in current instruction sizes.
Ergo, increasing register banks is a major architecture & platform change from hardware to software redesign, with heavy end user impact, and a fair chance of decreasing performance.
In contrast, anything that improves caching performance is a big non-disruptive win.
Then think of registers as just part of the fetch & store pipelines for staging operations on stack values.
Forth goes all in with this approach.
If you add more registers, the cost per register increases rapidly, and you very quickly hit your limits.
If you make registers wider, that's still very expensive, and you introduce extra steps to get to your data most of the time.
So no, you can't do that in a reasonable way.
2.5d is just too easy and effective - we're going to have lots more chiplets, and only the cool ones will get stacked.
remember, we're talking TB/s these days.