While these appear to be mostly guesses, there is an idea that Starfield will generate somewhere between 3-5 million Game Pass subs by itself. 5 million subs would be 20% of all Xbox Game Pass subs to date, as the last reported figure was 25 million in January of 2022.
Personally, my 3 years Gold to Ultimate ends in September. What I’ll probably do is play Starfield on release, and if I enjoy it, I’ll purchase it outright when my subscription ends.
I don’t think these “gaming subscription services” are worth it at all, even when my last 3 years have averaged £3 per month for game pass.