Not pay up front means pay later? I don't see how nostr works without paid relays. If users don't pay, eventually they will need ads and other revenue streams or they will be unsustainable.
Discussion
Reddit-like community solutions could also be envisioned, where you can join a relay and read/like/boost but not write unless you wait x days (anti spam) or pay a quick access fee.
At least people have access to quality content and load speeds from the get go, which helps attract new people. Paid 'members' could also have additional privileges in that specific client/relay