Replace? No. Be a valiable second option? Sure. Like in the early 2000 when you had dozens of major forums for certain topics. Something Awful, GameFAQs, Digg, Slashdot, 4chan, NeoGAF… It‘s not a natural law that there has to be one service having 95 % of the discussion market locked up.
Yes! Very much this. Imagine if lemmy would grow to just a few million users. That's the size of Digg when the migration to Reddit happened! Not everything needs to have a billion users and there's more engagement in small communities anyway.
Yeah. This makes me think of people who assume Tumblr is dead and unusable when everyone left, whereas in reality it has had a resurgence of creativity instead. Things like Goncharov happen because the people there still have a critical mass.
Platform don't die. They can flounder a bit, and I'm sure that even Reddit and Lemmy will one day do so too. But they're there.