In my case there's quite a few PS4 games I missed out on so while I plan to just buy stuff like Blade Runner, Ragnarok and Elden Ring, something like PS+ Extra or Premium would do good so I can play through some stuff like Detroit: Become Human, Until Dawn, SotC Remake etc. I actually didn't expect any PS5 1P games to be included as soon as they've put them there, I guess that is a nice bonus.
But as you say, for people who didn't miss out on these games, already played them or picked them up at some point before, you kind of have to look at these services in terms of what new content they are providing. I think Sony's taken a very different approach than Microsoft here, they're positioning PS+ as something of a great backlog at an affordable price, but it's clearly for older titles and with the classic systems, appealing to nostalgia. That other stuff is additive to the online, which a vast majority are going to need anyway so that's going to keep PS+ Essential as a priority for them.
MS's gone for a much different approach and I don't know how well it's going to pan out. I think we have a decent idea what the cap for the Netflix/HBO Max/Disney + services are, seems to be about 250 million. Spotify has 422 million. Those are mediums where consuming a lot of content per hour/day/week/month is MUCH easier to do than games let alone you can put lots of that stuff in the background, whereas with games you have to actively pay attention as you engage with them, they ask more out of the user in that way.
So for stuff like GamePass I don't think they have a potential market any larger than 80 - 100 million, and I think that could be a bit too optimistic. So in absolute best-case scenario MS would be getting $18 billion a year from subs if they all paid monthly a whole year for GPU. But as you just pointed out, that's not realistic at all, for many reasons, so it would be a lot less than that.
@Heisenberg007 and I ran some numbers months ago about GamePass on GAF when the Axios report came out, and plugged in some other revenue for other services and companies fitting the numbers in that report (even if that report was kinda bunk). We kind of came around $725 million - $750 million a year for GamePass off 25 million subs, based on that info, and that might've been slightly generous. I think given how young the game subscription model still is, if these services were pulling in huge numbers the leading ones would want to puff their chest a bit and let potential 3P partners know upfront how lucrative they can be, to draw in more of their big games to said services.
That's one of the reasons I think PS+ is going to surprise a lot of people, because Sony may not have reported PS Now fiscal numbers, but they regularly do for PS+, and this revamp is basically now PS+ and PS Now rolled into one. There's pretty good money there in PS+, and ultimately that's what publishers really care about, so I wouldn't be surprised if Sony can work out some bigger 3P deals into PS+ than Microsoft can for GamePass.