From a quick search, it seems that the operating costs for Meta in 2022 was USD $87.66 billion. Twitter was $5.6 billion in 2021. Reddit's is a mystery but it has 192 million more monthly active users than Twitter (not that they necessarily post the same amount of content or that the content is as expensive to store/serve, but in any case it's not going to be cheap either.)
So to run something like Facebook, you'd have to have one of the best-funded non-profits ever. Non-profit doesn't mean it has to be a charitable foundation (so you could run ads and sell services, which you'll need to, because you're not getting $87.66 billion through donations), but even non-profit companies with a decent amount of revenue do not come close to that amount. I don't think it's impossible to create an ethical, non-profit alternative to Facebook, but I understand why one hasn't been created (there are non-profit social networks, but most seem to be distributed/federated social networks, which are a very different experience for most users.)
Even if you created something with perfect feature-parity of Facebook, it'd still be extremely difficult to convince users to switch. For messaging platforms, there is a non-profit organization that has a secure and private messaging app (Signal) but few people use it compared to Facebook Messenger, Telegram and KakaoTalk (which by default don't use end-to-end encryption) or even Snapchat (doesn't use E2EE for messages aside from photos) (WhatsApp at least always uses E2EE, not sure about Line.)
> Enshitification requires the profit motive.
Not really. Firefox killed support for WPAs: that alone is enough reason for some to switch to a Chromium-based browser. They've done several questionable UI redesigns that their users don't seem to like (people like a consistent UI.) They're also starting a bad trend of adding new features that they're killing within a year or two (their password manager, their E2EE file sharing service, their encrypted notes.) I personally didn't use any of those features, but for any user who used those new features that they promoted, having to switch a year or two later is a good way to turn them off your products. Google is obviously far worse with this with their various services, but the point is that quality can decrease even when their is no profit motive. I'll agree with you that it's certainly more likely to happy when there's a profit motive though.