Yeah but DDR3 was faster then and still is now - I can't imagine my M4 Mac having a slightly older version RAM would meaningfully affect me, but another 24GB of it (or whatever I have) would certainly be an improvement.
It probably would. LPDDR5 memory bandwidth on M-series Macs is several multiples of the memory bandwidth available on DDR3 (depending on the exact chip, Pro, Max, etc... 4x to beyond 10x.) The whole system would be much, much slower.
I didn't say anything about USB? If you're referring to upthread wanting RAM to be standardised like USB (not standardised on something like USB), I took that to mean backwards compatible slots. (And maybe handicapped forwards compatibility, though why would you.) M.2/PCIe is this way.
Yeah, sure but... if it's real fast, lets connect SSDs to it and have real fast swap. Whatever it is will be limited to the bus speed. And that's gonna be silly compared to RAM speed.
CPUs have L1, L2, L3+ caches too after all.