No, people will not run large, public relays for free, forever. These relay are already gigantic and they're quickly growing. They eat up serious resources, and they are only performant if they are tightly managed and cleverly architected. Much (most?) of the traffic they receive is bots, spam, scammers, attackers, and scrapers.

Not to mention having to deal with the material people store on the relay and the possible personal or legal consequences of having that on your computer.

People don't run Bitcoin nodes for free, either. They run them to manage _their own money_. The way Bitcoin is designed, you automatically help other people manage their money, when you manage your own, as it is all on the same chain. And storing something on Bitcoin costs you Bitcoin. It isn't free storage.

Relays are not a chain. Almost everyone can and will only store and manage the notes they are personally interested in, and maybe those of their friends and contacts. That is enough, for basic communication (if your clients have AUTH and outboxes implemented), but larger relays facilitate wider reach. They aren't all going to go away... so long as they bring the person running it higher benefit and reward, than cost and bother.