At the moment, they're a bit on the "too big to fail" side. Digg is still around, despite much of the user base leaving for Reddit, and I imagine both Twitter and Reddit will still be around in some shape or form, even if Lemmy/Mastodon somehow make it big in the same way.

That's not even getting into things like how Reddit posts are still some of the more useful sources of information/discussion on the internet, due to the decline of forums and bulletin boards, so people will end up returning to it in some shape or form, if only to try and get recommendations/solve problems that they're having.

What might make them more likely to die is if they're not profitable, and they run out of money without being bought up, but that's less everyone leaving, and more the service shutting up shop overnight.

Which both parties seem to be trying to do in one way or another. Twitter is haemorrhaging money, and Reddit's recent controversies can't be doing good things to its stock price if the CEO more or less implied the company was not competent enough to make their own app profitable.