Most servers already seem to have Patreons or similar donation platforms running, and subscriptions would not surprise me as everyone starts to settle into this thing. It would make a lot of sense to help spread the load and since content wouldn’t be gated behind the subscription, I can’t see why it would be bad.
I think I have similar thoughts on ads. If an instance wants to run ads to support itself, I don’t see an issue as long as those ads aren’t “federated” out and sent to other places.
I think the ability to have all of these different setups, without restricting any access to content, is the beauty of the fediverse. At least as I understand it.