You can do your part! I set up a recurring donation recently, recurring donations are much better for the sustainment of an instance since the owner knows they can keep running and not have to worry about intermittent donations.
Larger instances will either have more donators or close their subscriptions if it really is unsustainable.
The more Lemmy grows, the more instances will show up, which will help spread the load.
You speak of sustainability but Lemmy survived the wave of incoming Redditors without much downtime. It's really impressive that this growth could happen. That's the power of decentralized systems: they scale!
Crazy-ish idea, but maybe Lemmy could make a feature where instances can have custom awards similar to Reddit gold and stuff, and users can buy them to both award posts they like and financially support the instance host
Users actually run a lot of larger instances on the Fediverse off of donations! I run a larger Mastodon server and we get literally double our monthly costs in donations. For every month we stay open, that is two months longer we stay open. Absolutely wild. We have about 1.5k users, with about 20-30 of them donating maybe $5-10 monthly. That's 2%-ish of the user base donating. Lemmy is LEAGUES easier to host given that it's written in Rust and is incredibly resource efficient. So I can only imagine it'll be even cheaper to host on donations.
One other thing I don’t see people pointing out is that Lemmy is basically a successor to classic web forums, so companies may run their own so they can have full control over the experience and moderation like for movies, series’s, or games, and in which case they would be more than happy to eat the cost of hosting. For example there could be a Lemmy.Disney.com instance to act as a Disney sanctioned discussion zone for their products where they could put tons of moderation rules in place.