I think there will be clients that offer relays with ads or as part of a subscription fee, might even be the Twitter company.
Discussion
wss://elonsplebs.twitter.com
My issue with ad-driven relays is that they invariably result in algos that drive people to the most contentious content in an effort to increase engagement and grow revenue. That's how Facebook and Twitter became the hellscapes they are now. So while I'm not against advertising in principle, with social networks it's never worked well.