I don't think twitter has anything to do with reddit -they don't exist in the same niche in social media. The sputter of mastadon was because the decline of twitter was largely short term outrage. I hear every day on hackernews that twitter is failing and I'm not sure that's true.

Twitter and reddit exist because there is a market for those types of social media experience. If even 50k computer science specialized people stop visiting reddit and twitter I don't think that matters to those sites overall traffic levels.

If you already aren't visiting twitter or reddit than you aren't really interested in the alternative for them, are you? Since you're already rejecting those experiences. You aren't the target audience, you're just imagining everyone is like you, when all evidence seems to go the other way.

So again, lets say people want an alternative platform to twitter, they could go to substack maybe. but whats the alternative to reddit? Discord? I don't think so.