I think Reddit will still be around, but when it comes to Twitter, the acquisition saddled the company with billions in debt amounting to around a billion in yearly interest repayment. Twitter already was barely making ends meet before that, and it only lost revenue for all the stupid decisions being made. Even if it was exactly as profitable as before, it wouldn't be enough anymore.
One day Twitter is simply going to stop working, or close off so much it becomes unusable, unless Musk personally tries to keep it afloat at expense of his own fortune, or someone is dumb enough to buy it off of him and do the same.
I also see a lot of wishful thinking in the Fediverse, but this is not the case here. People there may not want to move on, but it's not gonna keep it afloat. I already see a lot of people making their escape plans and linking alternative platforms every day.
It would be good if the change was towards Linux, but it seems more likely the transition will be towards an increasingly restrictive Android, reflecting how phones have become people's primary computing devices, so I'm not looking forward to it.