As with EA Play (formerly Origin Access), pretty much all, if not all, games on XBGP can be bought and paid for outside XBGP. In fact games come and go, much like Netflix content.
You don't need XBGP to buy Microsoft Flight Simulator for example.
At the end of the day it's a personal choice as to whether a subscription is worth having, either for the content or the cost/game time. I currently have one for EA Play (worth it for the games I play there) and one for XBGP for PC which I got to play FS, and which is probably not so good value for me right now. Next year I will ditch EA Play when my subscription expires as by then (hopefully) it will be included in the XBGP as previously announced.
You make your choice, you pay your money. Or you don't.