Is it better for the business to say "I sold 1m units" or "I have 2m monthly paying subscribers who played my game"? No brainer.
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.
Predictions (Made July 2019)
LTD: PS4 - 130m, Switch - 110m, XBO - 52m 2019 : PS4 - 15m, Switch - 18.8m, XBO - 4.8m 2020: Switch - 22m (Peak Year)