Well, this is where we disagree.
Two slots is definitely not enough for 99% of the users like you think. With larger SSDs costing more than two smaller ones together, many people use 2 drives already. Just have a look at the discussion that sparked here, when I suggested no RAID0 for SSDs:
https://www.techpowerup.com/forums/threads/rate-my-build-plan.272869/
As for SATA SSDs, I'm sure they'll not disappear over night, but they're not exactly thriving either. I mean, looking at Samsung, they released the 860 EVO like two years ago and more recently the 870 QVO (which is QLC, so meh). How many NVMe drives have they released in the same time frame?