At least with Lemmy there's lots of different servers, each with their own running costs.

Each could try a different way of keeping the lights on. Some could run on donations only, some could use small unobtrusive ads on the side, some could do lots of ads. If any server does too little they'll go down due to lack of funding, if any server does too much the users will migrate elsewhere, as it's quite easy to make a new account on another instance and keep following the same communities.

Even if we end up with some large-scale instances with big servers, millions of users and serious money involved, they won't have a monopoly on all the content like with reddit, so the competition should keep them from doing anything stupid.