There's not much Letterboxd does that cannot be federated. For one, their metadata database is a near full replica of TMDB, with very minor exceptions such as retaining deleted TMDB media. I'm not opposed to Letterboxd dying if it means people consider an alternative that is less fragile.

The value of many platforms is not the database or the stack, but the community on it. People use Letterboxd because their friends are there and because actors and directors have their lists and ratings there.