You think there is little cost, but that is because the costs are hidden from you.  The relay network is neglected and shrinking, and along with it, censorship resistance.  The issue with nostr, in fact with any social netowork, according to research, is that after some time there is a tendancy to free ride.  After that kicks in, cooperation decreases and there is a force to reduce.  Nostr is not inevitable.  It's rivals have grown 10x in the same time nostr has gone sideways, and the relay structure slowly dying.  It's a double loss, because you lose the relay, and you also lose a talented developer that might have helped the project.  In fact the cost of developer time is more than the cost of supporting the relays.  The problem with nostr is that the relays were intentionally starved of support, with the comment, "we dont need more relays".  Which is sad.