For the time being most people writing to paid relays also write to the public ones, and multiple clients read from paid and public (just not on Global), so new folks who are only using public relays will still be seen and interacted with. But the public relays need to figure out the spam problem or they won't last. It's a tough problem to be sure (I battle with it to keep NostrGram's database from being overloaded by it) but unless a solid solution is forthcoming they'll get buried in spam.