To support decentralization and spread, should lemmy.world close registration at some point to prevent a performance overload due to too many users? Of course, if registration is disabled, there could be a hint placed somewhere near that from other instances you can interact with content on lemmy.world just like you had registered on it. There could be a link to join-lemmys instance overview.
Not really. Also by your logic you can’t trust anyone ever because there is always a risk they turn bad at some point in the future. All we can do is evaluate what we have in front of us at the moment. Current evidence suggests Ruud is trustworthy, committed and capable of running a large Fediverse instance.