I'm not sure why this is self-evident. The attach rate on Windows 8 apps was quite low. The store was essentially DOA within 6 months of launch and never revived. Paid apps instantly failed, and unpaid apps failed when advertisers stopped buying advertising (because no one was using apps), leading Microsoft to directly subsidize advertising, which they eventually stopped doing, marooning all the developers. If what I'm saying here is news to you,
start here and Google around a little. This move came at the same time they started
paying people to make apps, which also didn't work.
Now, Windows 10 has some stuff going for it app-wise--and definitely some extra stuff going for it in terms of games--and it's possible the W10 app store will be healthier than W8. But I don't think it's guaranteed, and I'm not sure why you feel it is as obvious as you do. It's not clear to me that if Windows 10 had a billion users, the W10 app store would have more active users than Steam.
(side note: the Mac App Store is also very dead across the board; is the parallel because both app stores were poorly executed, or because there's less of a demand for an app store among normal OS purchasers on desktops, where the web browser already does 90% of what people want to do, than on mobile?)