It might still be used heavily, but Microsoft has stated that they will be investing more fully in HTML5. Silverlight won't last.
HTML5 has gotten to the point where I think we can safely assume there won't be major new features added. Meaning HTML5 won't be able to...
- Record audio and video
- Support advanced video features like overlays
- Support pay media features like tracking, DRM, etc...
- Support out of browser experience (though I suppose each browser could implement this)
- Support multi-threading (though I suppose each browser could implement this)
- and others I can't think of off the top of my head
So either Flash or Silverlight will still have a future long after everyone's using HTML 5 based browsers. The question is whether Microsoft can capitalize on Flash's diminishing position and use successes like Netflix to supplant Adobe.
(Which is why I said Microsoft is being stupid by not giving Silverlight more support)
Synchronize javascript events to keyframes in the video?
I'm not sure what you mean by "Record audio and video", but there is a device access API for webapps to get to webcams, mics, video game controllers, etc.
Mozilla's audio api also allows Javascript to generate raw audio data, and I expect (well, certainly hope) that this will be adopted by the other browsers as well.
Video overlays - I'm not sure what you meant since there are a few different terms like this. If you mean routing the video data in a certain way, you're right, I don't see that happening. If you mean overlaying one video on top of another (or anything like that), that's already possible - check out http://html5doctor.com/video-canvas-magic/
The rest of it you may be right. DRM will have to be browser-specific, and it's likely to fit in at all with an OSS project. Multi-threading seems like it would likely require a change to the Javascript implementations.
As for your other points I don't disagree there are other options from the various browsers (Mozilla being the most aggressive) but they aren't part of the standard and will be browser specific. The beauty of Silverlight is that the plugin works in every browser in the same way.
What about Web Workers?