There's apps out there developed for MR devices such as the Hololens, it's coming. I can already get a MR HMD from China for under 100, there's apps that have been designed for it. It's rudimentary, but it's happening.
As for 3D via HMD for SL, you can use that already with Android, the app is called Lumiya. It's somewhat rudimentary, but it does work and it's proof that LL doesn't need to be on board with this, it will happen.
The WMR headsets are not actually MR, the cameras are designed for tracking and depth perception, they are not very good visually. It's more of a budget entry into VR. They are not a standard screen though, which means any SL app has to be developed specifically for the MR platform. This may or may not be to our benefit.
There's one thing that is certain and that is that there is demand for this. There's social apps out there for "chill and build". WMR is also compatible with Sketchup, it's actually remarkably easy to export someone's creation and move into it, using it instead of the cliff house. You can also download environments for SteamVR. Both of these allow friends to visit and you can play games together. It took me no time at all to pick up and work with Valve's hammer, their editing app which allows me to build a "level", an environment to use within steamVR.
Let's hope LL gets on board with this as SL has advantages as it is already established. If not, you can be sure someone will write an app for it. Just keep in mind that it will have to render the surrounds of the player twice and at either 60 or 90hz. The lows are going to be significant.