I think there will be more incentive in the future. For now there are only so many users, the network seems to be able to sustain its current operations. Over time clients will continue to evolve and accept that many relays exist around the world, and users might want to keep visiting them. Operations will continue to improve across the board. With this in mind, the interface for renting a relay from another party is far behind. Parallel this issue, clients are still trying to adopt outbox model. Working outward from "functional outbox support" is a bit of a climb, but I think Jumble.social is doing this right. In the settings you can create relay "collections" which double as custom feeds. This means relays can expand to support a more diverse landscape and userbase. Once Nostr has a good footing in these regards, I think a lot of the headaches will quickly dissipate. Free relays will likely always exist, but maybe they aren't particularly valuable to established users. That's how I see it anyways.