From Sean Chan at MSPowerUser:
Unfortunately, as it turns out, Windows Mixed Reality won’t support content from SteamVR on its initial launch. According to German site ComputerBase, Windows Mixed Reality’s communication director Greg Sullivan stated that support for SteamVR won’t be available on the launch day. In fact, Sullivan mentioned that the companies have just started working on the integration — and it’ll probably take a while before Windows Mixed Reality users can actually run content from SteamVR on their new headsets.
When it comes to gaming (or any other software) development, an old adage is that you should always buy a product for what it has now rather than based on promises of future features. Despite the number of software launch partners Microsoft announced, the lack of support for SteamVR’s large catalogue from the outset has just made the product a lot less interesting, and will probably mean I will hang on a few months to see if it will really materialize before spending hundreds of dollars.Microsoft had already walked back the SteamVR announcement a bit by announcing that the new MR headsets would require Windows 10's Fall Creators Update to work (creating an obvious incompatibility with SteamVR, which has no such restriction), this isn't too surprising, but it is a little disappointing. It's difficult to imagine what sort of MR/SteamVR integration will even be possible, as long as Microsoft are determined to use MR as a tool to drive adoption of their walled-garden Windows 10 ecosystem... something which isn't in Valve's interests at all.
For VR, it's really the worst of all worlds. The first generation of hardware will be rendered obsolete by the new MR headsets, but the new MR headsets won't be able to build their user base, either, thanks to Microsoft's pathological monopolistic tendencies. The Fall Creators Update is already launching a month late (in October, and not on-schedule in September), and if the Anniversary and Creators Updates are any indication, the FCU will take months to be "fully" rolled out. Making MR inoperable without the FCU puts any potential MR customer in a position of having to wait months for their OS to be updated enough to even work with the hardware... and then more months of waiting for the hardware to be compatible with the biggest available library of VR content. It's idiotic. Microsoft literally couldn't have planned it any worse.
But, then, we are talking about Microsoft, here. We really shouldn't be at all surprised, at this point.