It’s true that several instances (the ones hosted in Mali?) closed, but I think you’re premature in saying the smaller instances are disappearing, Lemmy.one included.
Sure, they’re bound to crash or go down for maintenance from time to time, but that doesn’t mean they’re finished. So far my experience has been that if you give it a couple days they’ll be back. Keep in mind these are shoestring operations and sometimes seeing major influxes of users.