Possibly, but they might increase because spammers won’t mind paying a few sats to spam the global feed. Eventually, they will probably charge a few sats per note you post.
Discussion
I doubt that business model would gain traction. People like free once paid for. Subscription, maybe.
Spam is def set to increase
Yes, it just depends on how the spam moves. There will be many different relay options. Some will be pay-once, others will be per-sat, and some will be free. They may even find other ways to monetize. What you choose will depend on what experience you hope to have.
For now though, definitely grab a paid relay so we can see your notes in global 😂