The distributed nature of Lemmy should make things more manageable. Personally, I'm running an instance on a dedicated machine I already pay for, so it's not costing me anything unless storage skyrockets. Many other instance hosts are also hobbyists that don't mind covering the costs, and may take some form of donations locally on their sidebars.

There probably should be a built-in feature for instance admins to enable a local donation button to contribute to their costs, though. While Lemmy is fairly resource-efficient, larger instances are eventually going to require pretty beefy VMs to keep up with the traffic, image uploads, etc. I could see some instances randomly vanishing when their owners can't/don't keep up with their bills (which would force users over to other instances), but ideally if any instance owners can't afford to cover it, they hand control over to another community member to pick it up.