Even when playing the long game, it doesn't make sense to alienate the pc crowd first by building an ecosystem that goes straight against their interests and wishes. As long as Microsoft keeps pushing the console experience, I don't see many pc gamers liking the Windows Store in two years. I could be wrong of course, time will tell.
You are overestimating Halo's success on pc tho. The first two Halo games didn't set the world on fire on pc, neither did other console-oriented shooters. Gears 4 for example was a huge flop on pc. Even in the weeks after release there weren't enough players to fill an online match, despite the fact that the game was given away for free to people buying a Nvidia gpu.
What Microsoft needs to succeed on pc is an Origin-like storefront, selling moddable and pc-focused games at reasonable prices. They still have a long, long way before achieving that.
I don't think that they "NEED" mods in order to be a successful storefront. Yes, there will be some that don't want to buy those games because they can't mod them or get mods but I'm sure that there's a bigger audience that just want the games than not have them because of no mods. That's my opinion. The prices thing, I agree with, I think digital games should be cheaper but that's MS trying to be everything to everybody *shrug*.
I'm not overestimating Halo's success on PC. As a gaming division, you can't have your premier title not work correctly on a system that you are trying to push towards. It doesn't have to set the world on fire like PUBG or something, that's not what I'm stating. They just need to make sure that that game isn't a failure because of their system, not because of the content of their game.
You stated that it doesn't make sense to alienate the pc crowd by building the system that they did...that part already happened and it they had their reasons for it, no matter if anyone agrees with it or not. Also, you'd be surprised how many people want a "Xbox GUI Mode" on their PC...I've seen that more than I thought I would over the years.
But just as you said, time will tell.
Perhaps. But pc gamers already have all of this for years. And Steam still has modding and much cheaper games thanks to free key generation, also something that Microsoft doesn't allow in the Windows Store. Why would anyone use the Xbox app instead of the ecosystem and apps they are used to?
You ask why anyone would use the Xbox app...well, that's on the person to decide...a reason I could suggest is communicating with friends who are on Xbox. Also, PC gamers use a multitude of things, be it Steam, or Skype (when it was more relevant) for voice comms, or Discord. PC isn't just Steam's ecosystem, it's whatever the user chooses to use (ya know, the beauty of being open). As I recall and correct me if I'm wrong but Steam doesn't have a Tournaments type of service and I know that they don't have Twitch nor Mixer integrated (outside of third party stuff). But I mentioned those things that they already have to say to the other user that MS already has a Steam competitor. To me, MS doesn't have to "beat Steam", it just has to be able to be launched beside it like Origin or GOG games or UPlay or Battle.net.