Exclusivity is a two-edged sword. What will happen to Steam if Valve locks others out of their market, but then one of those others produces the magic ingredient that makes VR really work well for the vast majority of people? For example, what if magic leap's multiple virtual focus planes turns out to be simply that good.

This paragraph indicates that Valve seems to appreciate that no one company has really put together the whole package yet. If they know that, then perhaps they will be smart enough to open Steam's VR to the open market, even if that means supporting competitors like Oculus.

At this stage, VR could still implode again like it did in the nineties. The smart move is to do what it takes to ensure there is a VR market a few years from now, not hamstring yourself worrying about who gets the biggest slice.