And on the MS store with crossbuy no publisher is going to charge people who buy the game on PC less than those who buy on their Xbox.
Prices for digital games on console are typically much higher for longer than people are used to on current PC stores/sellers, so if it becomes the case that a game is locked on PC to the MS store, for ~£50 new and for a long time to come, with the odd sale that knocks it down by 10%, buyers used to Steam's prices, steady lowering of price over time and big sales are going to scoff at console prices even if the game is exclusive to MS store.
For example,
Ryse - console exclusive:
https://store.xbox.com/en-GB/Xbox-One/Bundle/Ryse-Legendary-Edition/8efcf807-c12d-440d-a755-5d7331fb60b9
http://store.steampowered.com/app/302510/
MS store: £45, Steam: £35.
COD: Advanced Warfare - multiplat:
https://store.xbox.com/en-GB/Xbox-One/Games/Call-of-Duty-Advanced-Warfare/40bbfa62-064c-4ff2-a861-dd82c109dfab
http://store.steampowered.com/app/209650/
MS store: £55, Steam: £40.
If this continues to be the case on Windows 10 with crossbuy, exclusive to the MS store or no, Microsoft will either need to negotiate lower prices with publishers on both their ecosystems, or watch as PC gamers buy their games anywhere else.