Between the current games they buy exclusivity for now, and their first party games, that would be a pretty steady flow of new content to keep people subscribed, combined with a solid backlog to attract other subscribers. I'd argue the potential is there for much more money, especially given the relatively lower attach rate. I think your faulty premise is the assumption that people will just subscribe for 1 month for the 1 game they want. But if you have a steady flow of content (whether new day 1 releases or high quality/popular games that aren't new), people won't stop subscribing. Yes there will people who do that, but it'd be far from a majority, and most of those people wouldn't be the ones willing to spend $70 for that game.
If people aren't mass unsubscribing from Xbox Game Pass, why would they from Playstation's when most people agree they have the better exclusives? The reason they won't do it is because they probably can't afford the initial losses that a subscription service like Game Pass requires, but given their quality exclusives and extensive backlog (which was way bigger than Xbox's pre Activision buyout), a true Playstation Game Pass competitor would be absolutely massive and almost certainly make them more money than they're making now