The problem is there's entire communities that, for better or worse, have (or possibly soon had) to live and die by Twitter because it's (unfortunately) the best platform for their needs.
Like look at artists, for example. Are there other sites out there to post art on? Absolutely, no one is denying that. But Twitter is the best site for allowing artists to self-promote their work, grow and interact with their audience, etc. If/when Twitter goes under, those people are completely fucked because they'd have to try and regrow their audience from scratch on a new site that likely only has a fraction of the audience of Twitter (and that's not even to say if whatever platform they move to even has the same level of interaction they can do on Twitter).
Hell that's not even getting into just trying to maintain contact with friend groups, news, etc. on the site and whatever happens to it in the future.