I think while Mastodon and Lemmy don't solve those issues particularly well at the moment I'm still confident they are solvable problems.

As for costs, I don't think it'll be that bad. It's not nearly as expensive if you're just trying to cover expenses (and not focused entirely on growth and revenue), and if a server does get to a point where the admins are concerned about donations keeping up they can cut off sign ups. Push incoming users to other instances that can handle the extra load (or spin up new ones if no more remain). It won't be the cleanest process and the inconvenience will make it tough to capture a lot of the potential incoming growth but Lemmy doesn't need to chase that growth entirely. It can grow at its own pace and handle what it can handle.