"no clear incentive" argument doesn't cut it, never did.

There is 100m self-hosted sites on the web, and owners _pay_ to host every one of them. Their incentive is their business/hobby/etc. On Nostr, creators/businesses have the same incentive to run their own relays and to pay for good relays to have their notes delivered to followers. There would be at least 10m relays if Nostr got widely adopted, not counting all those phone relays serving as local caches.

Creators pay for web hosting. Creators will pay for nostr relays/media-hosting/etc. 

I know free social platforms made this a bit non-obvious, but please look around the web. Hundred million people and businesses pay to have their own online space. Nothing new here.