You don't start with 100 million users because Gamepass never had 100 million users. There are 25 million right now.
And many of those 25 million aren't even paying those $10 / month. Some of those are $1-per-3-months subscriptions ($0.33 /month), some are 3-years-Gold-converted-to-Gamepass-for-$120 -
like me - ($3.33 /month until 2025), some are even using VPN to buy Gold on the Turkish store for like $30/year to make it even cheaper.
When do you expect Microsoft to have 100 million users? Gamepass has been a great deal for consumers since it released in 2017. But even during the excellent conditions of the pandemic, with government-enforced isolation periods and people stuck in their houses,
Gamepass rose 66% in the last 15 months.
With a ~53% growth per year, it'll take until April 2025 to reach 100 million subscribers. Assuming all these purchases compensate for a general decrease in videogame/electronics expenditure if/when people get to leave their houses.