I don't think that would be a smart move. Considering Steam is one of the biggest (if not THE biggest) gaming hubs for Windows and other platforms, if they blocked this and somehow forced people to use the Store/XBox instead, it could backfire and lead gamers en masse to the Steam OS and hardware, for example.

These "migrations" of users from one service to another should happen naturally to be successful. Not enforced because of platform restrictions, but based on these new platforms being actually better and more desirable for the end user.