All subreddits are run by Reddit; if Reddit decides to overrun it with ads, require you to use their app, make content impossible to enjoy, or incorporate some awful AI bullshit, nobody can really do anything about it.
Over here, you are in charge of your own user experience. You're reading this content from dbzer0; I'm using an entirely different application called kbin. We have completely different user experiences, and some users might be banned on my server but not on yours (or vice versa).
Others might get different user experiences through apps or front-ends such as Old Lemmy or more experimental stuff. It's basically going to be a lot more difficult to enshittify as everybody is chosing their own experience.
As for the communities, they are indeed at the mercy of whoever runs a particular server. If the lemmy.world admins go a bit crazy, users might for example respond by jumping ship to the !fediverse community on a different server.
On the contrary, a bunch of scattered communities create one large user base. The people you see in this thread all come from a bunch of different websites and services. You'll see users from startrek.website discussing woodworking in communities hosted by lemmy.ca.