It's better to have them forget about it and have the server and its communities locked than it is to have them left open to abuse in your absence. At least in the case of the locked one they can still use it on their communities on other instances (I see no reason to lock existing users, just registrations, community creation, and maybe posts and comments on hosted communities).

Having a team of admins is another good solution to this problem since if one disappears the others can still do the work while they are gone.

Yup it's a proxy to circumvent defederation, they're just posting shit there because at least when they started it was federated with many others. They'll probably just flee elsewhere but this is why it's very important to make sure you have people to manage the instance in case you aren't able to anymore.