This already is a thing, see StarTrek.website, Lemmy.film and LemmyNSFW.com for just a few examples.
The admin paranoia is tough because it goes both ways. Yes, big instances mean huge losses if they get abandoned, defederated or the admin loses his mind. At the same time, smaller instances are often run by regular people who can’t or aren’t interested in running an instance long term, or don’t have the ability to have 99%+ uptime.
At the moment momentum is important to reach critical mass of users and it’s just not feasible to tell most Reddit refugees “Okay so go to look up the handful of instances closest to your geographical location. Research their uptime, the reliability of their admins, their funding, their rules and whether they’re defederated from any major instances. Just pick the one that best suits your needs! Oh, also remember to use !community (don’t forget the exclamation point!) when searching for communities because many small instances haven’t connected yet to everyone. This also means these subs will appear empty at first because old content doesn’t get propagated but don’t worry about it. Oh and also sorting topics by Hot is bugged on small instances currently.”