>>64174314 (OP)
Honestly I've thought through the whole modular meme before and hell even swapping uppers on a single lower is still a pain in the ass if you're changing calibers or swapping between differently gassed uppers. Unless you're too poor to spend the extra $200-300 for a lower or live in a shithole that limits your firearms, it's simply not worth it.
Let's take your example here
>Two different buffers at a minimum if not three are needed and probably two different springs as well for 9mm, 7.62x39, and 5.56
>Limited to a mil-spec trigger or at least not Giessele so the 9mm doesnt snap your hammer
Just for fun pretend you dont care about any of that. You can still achieve this all with a standard lower, but would need to compromise on magazine choice. So regardless you still get your various calibers that this lower proposes to solve.
I rather just have three complete firearms for the extra cost of $1k or so than play musical buffers and worry about damaging my trigger personally