I expect relay runners to give up at one point. I see relays going down all the time. Some never come back. 

What kind of incentives exist for keeping a relay up for 30-40 years?
None.

In social media users want to see the posts they’ve done and the reactions. With nostr relays that is not working out reliable. 

Your posts, answers, images etc. could all be gone tomorrow and maybe come back the day after or never come back. Maybe the relay which connected you with a friend goes down and then no nostr app shows your friend profile anymore…

That‘s my thought for the social media part/usage of the protocol… which is unusable for ‚normie‘ social media users (normies = not interested in decentralization).