It won't reach 100m.
1. Xbox consoles never reached 100m. Ps5 was just above it.
Contrasting console sales against a multi-platform subscription service is disingenuous. Xbox doesn't need to sell 100m consoles to have 100m Game Pass subscribers. Hell, Xbox doesn't need to sell any consoles to have 100m Game Pass subscribers if taken to the extreme. This is the point of Game Pass. I presume you also meant PS3, as PS5 has just released this past year, and PS4 passed 100m a while ago.
2. People on mobile don't like console or AAA games. You think with their expensive iPhones they wouldn't have bought a console from previous gens already?
Demonstrably wrong. The only objective fact is that people on mobile play games available on mobile. This is why there are mobile ports of GTA and Final Fantasy, and mobile versions of Fortnight and Call of Duty that have all been enormously successful.
3. People who can't afford a console won't provide that much $ anyways.
Refer to point one. Game Pass like-services are designed to circumvent the need for expensive hardware outlays, thus lowering the barrier of entry to the ecosystem. People who can't afford a console still likely have a Desktop computer or mobile phone where Game Pass is also available. For a fraction of the cost of a retail game, they can have access to hundreds of games on their existing platforms. This also ignores the fact that the Series S is the cheapest next-gen console, and is referred to as a "Game Pass box".
4. People will eventually run of things to play because of their limited subjective preferences in games and drop the service.
As you may not be aware, Microsoft has recently invested approximately USD$77 billion dollars in acquiring two publishers, bringing their total internal development studios to around 32. In addition, Microsoft continues to sign third party games for timed appearance on the service each month. The games on Game Pass cross every genre. Microsoft has been quietly cultivating Game Pass with an enormous breadth of titles, and have boasted about the service's ability to service new title discovery. The point isn't to give RTS players unlimited new RTS games, but rather to entice RTS players to try non-RTS games because they can play as many as they want as part of their subscription. According to Microsoft, it's working well.
5. People will prefer to buy 3 games for the same value because that's all they need for the year. (Casuals)
Using Sony as the example, three PlayStation 5 games costs AUD$375.00, at AUD$125.00 a piece. A year of Game Pass Ultimate, covering PC, Xbox, and xCloud, only costs AUD$191.40 if paid at full price on a month-by-month basis. The cost drops considerably if an annual subscription is purchased, or other deals and exploits are used to lower the price. The "casuals" you're referring to are likely just buying the biggest games - Call of Duty, Madden, and Fifa. Call of Duty was recently acquired by Microsoft and will be included in Game Pass next year, and Madden and Fifa's legacy titles are all available via Game Pass Ultimate's inclusion of EA Play. So, a casual can buy a year of Game Pass Ultimate paid per month and still have over AUD$180.00 in their pocket to buy a couple of retail games - including a brand new release that isn't on Game Pass.
The limiting factor in Game Pass's ability to achieve 100m subscribers is Microsoft's ability to consistently deliver industry leading titles on the service month after month after month. Games like Forza Horizon 5 are a great example of what they need to deliver, while Halo Infinite is an example of what they need to avoid - poorly made bloated games designed as "platforms" instead. We can discuss the ability of Microsoft's developers to achieve that goal, and the incentives they may have to turn everything into a GaaS that will hinder them, but, basically every reason you've stated so far is wrong.
