When I first heard these changes, I seemed to hear a lot of apocalyptic stuff - "They are getting rid of the SDK". Having heard a little bit more, this sounds perfectly reasonable - Altspace need to get rid of the Coherent plugin (that also seems to eat up a lot of processes on Windows). I hope that the overall model for SDK apps / extensions is retained with something else, for me this seems to be by far the best way to develop apps, even if it is extremely challenging, purely because waiting on Microsoft to provide all the tools is a bottleneck, and hosting the content is going to be a challenge. Microsoft really should use the fact that Altspace is web-based for extensions to even push .NET.
The SDK is by far one of the best additions I've seen to a social VR platform so far. If I had a few more requests, dynamic lighting would be up there, and perhaps native integration with the physics engine, but everything else was fine. I'm not an A-Frame user I prefer three.js, but it was extremely fast to get started. I will try to document all the demos/apps I built before shutdown.