I just ran across this bit of news
So it’s not just Mac Pros and Studios that can get SSD upgrades. The “iBoff” repair channel just demonstrated a proof-of-concept system for doing this on Apple Silicon laptops.
Remove the old flash chips and then solder down a board that installs an M.2 socket in its place. You can then install a custom shape M.2 flash module (needed to fit in the crowded Mac case) for 512 GB, 1TB or 2TB of storage.
It won’t be cheap, and you need to run Configurator from another Mac to pair it with the Mac after installation, but it proves that it can be done. And performance is the same as Apple’s soldered-down storage.
Which also proves that it’s only a business reason, not a technical one, why Apple is soldering down storage.