For the pure "social feed" the scope of relay-centrism is small, I think (you can browse notes in niche relays "global" feed, or you can filter replies in threads by relays or groups of relays to gather different views of any given note -- what else?), but when it comes to other Nostr use cases I see relays being necessarily more and more at the center of the experience.
Discussion
Could relays become sub culture hubs, paid news services, streaming services and so much more. By selecting a relay you would chose a set of content creators and the community that loves them.
Hopefully things don’t go the way of noted open protocol email, where the Gmail relay/client hybrid dominates.
As long as it remains open I think it's fine, but there will always be the chance of a superapp capturing the network and the protocol, like Google has done multiple times.