MS will take a 30% cut of every game sold in the W10 store as well.
Why would a developer put his game on the W10 store that everyone hates and doesn't want to use instead of Steam with its 150 million potential customers?
Right now Microsoft is not asking that, they're just putting their games in their store, indeed the "issue" here is people complaining about that. When and if Microsoft will try to bring other developers on their store what they can offer is the Xbox Live infrastructure, Azure servers and i wouldn't know what else could they try