How is it a "no brainer" there's many factors involved.
2m subscribing for one month is $20m. 1m purchasing the title at launch would be $60m. And let's not forget game pass gives you access to 150+ games so it can be said that not all that $20m is going to sea of thieves assuming they played any other title on the system that month.
There's a vast amount of information we don't have to make a "no brainer" assumption. How much of the gamepass revenue goes to third parties who put their games on the service for instance.
The fact is even if MS received 100% of the revenue, you can subscribe to game pass for $120 a year, so if MS release more than $120 of content that you would have bought at launch, 2 games basically, they're losing out on money, and that's assuming you have no breaks between subscription months also.
There is a bigger picture at play. Its the future of gaming and its a certainty. Games as a service akin to Netflix and Spotify is guaranteed.