And no, not until it's more well known and popular will it be a system seller. The average gamer still mostly has no idea what GamePass is or how it works. Years after EA Access launched you'd be amazed at how many people still didn't know about it or how it worked. So many times I'd see people about to buy a year old Madden or Battlefield for $40 and I'd ask them why they wouldn't just get two years of EA Access for that price and have access to that game they were about to buy and tons others and they had no idea it existed.
If you already own an Xbox console then you know about it because they market it very well on the console and in the store. But new buyers, meh, not quite yet. Imagine what they can do for the next gen launch though.. if they manage to snag a big third party launch title on GamePass, that would be a major selling point. I already have GamePass Ultimate though most of 2021, so when I buy the next Xbox I already get Halo Infinite and any other MS launch game essentially free. It's incredibly pro-consumer. But still needs time. Maybe once older Xbone hardware gets very cheap.