What you say is true for most kind 1 clients. Your relays just serve to help you be censorship resistant with your public posts by having them stored on a few relays for redundancy. But not all clients are kind 1 clients, are they? For one, there is #Flotilla which is built specifically for relay-based communities. So you can run a relay that is just for a specific community, and white-list anyone you want to give access to the community. Posts to that community relay can either be visible to anyone who is considering joining the community, or only visible to community members, based on the read permissions of the relay. There is also Jumble.social, which is a web client that puts relays front and center, and allows you to post specifically to a single relay, if you choose, without having to change your main inbox or outbox relays. I highly recommend checking it out. As for the question of whether more public relays are needed, you can judge that for yourself. Hop over to https://next.nostr.watch and take a look at how many reachable relays there currently are on the Nostr protocol. One more is absolutely welcomed to have more options available, but ideally we should be moving to something of a modified outbox model, where most users are posting to small and even personal relays, rather than large public relays, and clients look at a user's relay list to determine where their notes need to be requested from, if they aren't readily available on the selected aggregation relay.