It was up 7 million in one year, it's growing faster every year, not at the same rate.
But normalized it has been 5 million per year; growth exploded in 2020 due to C19 lockdowns but slowed in 2021.
That's before we even think about Activision and all the stuff we know is coming in the future (Bethesda games, Fable etc).
TBH I'm not 100% convinced many of the big Activision games (COD, WoW potentially) or even some of the big Zenimax sequels will go to GamePass Day 1. For COD in particular there's no business reason to do so. They would be giving up guaranteed sales revenue & profit and potentially compromise sales on other platforms like PS and Nintendo for a possibly modest growth in GP subs (particularly those who stay subbed the whole year) and less money, would that be best for them as a company to do?
They'll probably definitely do exclusive maps, skins, weapons etc. with COD through GP subs, stuff like that. Add the older games to the service, too. But Day 1 for new CODs is not happening.
Elden Scrolls VI alone will be enough to bring a lot of people into the fold. 100m is just a sample figure anyway, nobody here knows what they have to reach to be happy and that equation will be unfathomably complicated with every factor at play. But that will naturally include things like (as an example, may not happen) rolling WoW subscriptions into Game Pass. They're all-in on it, they've made that very clear by now.
TES6 will definitely bring in some attention, but it could also have a more mute effect like Halo Infinite is having now, really depends on how Starfield hits. Halo should be doing a lot better than it is right now, they'd be seeing steady big monthly growth if it was. But, 343i have completely dropped the ball (again), hopefully there are some massive changes going forward because they need to be much better at keeping a consistent update pipeline of content & features.
I don't see how they roll over WoW subs without an extra tier; that game already brings in 4 million - 6 million a month at $15. That's 1/4 the number of GamePass subs paying $15 a month for a single game. Do you sacrifice that by rolling them into GamePass at no extra cost, or would it be better to add a $20/month tier that includes WoW or a cut-down version of WoW?
Point is whether it takes 5 years, 10 years, 20 years, it will be sustainable, and video games aren't going anywhere fast.
No, they aren't going anywhere you're right about that. But the industry will keep evolving. What if things trend away from everything being driven by subscription services? What if the subscription services market collapses due to fragmentation and eventually offering no real value savings for people who might need to sub to multiple services to get the content they could've simply purchased individually before? What if that collapse is spurred by less incentive to put games on discount sales prices at traditional intervals, offering net negative value to users who might've purchased multiple games at a discount a little ways after launch, because those publishers then want people to sub to their service for those games?
A lot of companies want to genuinely believe game subscription/streaming services are the future but they might be surprised if the core market ultimately rejects it due to things like those I just mentioned above. And these companies still need the hardcore/core market to support this stuff for it to actually work, or at least get off the ground in a stable way.