expectation that donations should cover admins labor costs
Not quite that. The argument is that admins shouldn't be treated as a disposable entity who don't need any money just because they are not directly asking for it.
It's a "I want you to want to do the dishes" kind of thing.
Shut the instance down, find someone to help you maintain the instance, or pass the instance off to someone else.
Easier said than done. We already have a long list of instances that disappear suddenly because the admins burned out, and I have had long discussions with admins from other instances who keep begging for more donations every month instead of just saying "you know what? You don't want to help me, so I don't owe your lot anything".
Mastodon because it is the largest number of instances and users, but the principle is the same. It hasn't happened with Lemmy yet because this subset of the Fediverse is ridiculously small compared to Mastodon/Pixelfed