> the flash controller is internal to the M1 SoC

Okay, it was part of the T2 chip before. And they were able to put replaceable SSDs into the Mac Pro. Those only contained the NAND chips and were proprietary, but still.

The problem I have with soldered SSDs is that flash memory has a limited lifespan in terms of writes. Once you're out of writes, then what? This is aggravated by the fact that M1 can't boot even from external media if the internal SSD isn't working.

It's worth noting that Apple's SSDs tend to be on the high side of the industry for write endurance. I think their calculus is that no reasonable user would exhaust the lifespan before the rest of the computer is well and truly obsolete.

(This was briefly not the case with that Big Sur SSD thrashing bug, but that's been fixed now)

It's actually split. The NVMe controller is in the M1 SoC, but the flash modules contain their own dumber controller and connect via PCIe in the back end (using a custom protocol, not NVMe).

Apple could and have put their SSDs in modules - that's how the T2 Mac Pro does it. They're still proprietary though, of course, due to the split design.