The problem is motherboards typically only have 1 or 2 NVMe ports, but 4+ SATA ports. So consumers will be losing a lot of potential storage expandability when SATA SSDs are gone, especially in small form factor cases that don't have room for 3.5" spinning disks.

No they're not. SATA isn't going anywhere. Hard drives are still the only option for truly massive amounts of affordable storage, and they won't be fitting into the M.2 form-factor anytime soon.

If you want the middle ground, fast but not as expensive as NVME, SATA SSDs make perfect sense. Put em in raid and you can get speed, too. Not just size.

Not to mention that plenty of still perfectly usable motherboards and laptops still only have the one M.2 slot, so if you want more than one drive, any additional ones have to be SATA.

This WILL reduce supply, which WILL increase prices. For M.2 drives too, because people who could have gone straight for SATA, now have to buy M.2 and put it in an adapter.