MS just came out with Silverlight 4 not too long ago, and it is based on the XAML way of doing things. Too, they just released Lightswitch, which I think is a way to integrate data driven mechanics into not only apps, but Silverlight as well.
I'm not sure what you mean by the support of Windows 8 with HTML, etc., other than to assume that the OS will recognize and allow for the rendering of HTML pages (without a seperate browser). If that is the case, it isn't so much the death of a particular technology, but that the OS will be more prone to have problems. By that, I mean not everyone writes, for example, javascript the same way - and there is a lot of deviation from "standards". So, if an OS doesn't handle things right, when it renders a page to your screen it could very well lock up with bad code.
Right now, the OS just passes the script or HTML file to the browser to deal with.
Personally, I find this type of a bundling concerning as it makes the difference between what is out on the web/network nigh indistinguishable from what is local to the machine. Heck, so many people now have a problem just figuring out how to log into a website - much less calling the helpdesk with "it's broke" and not knowing what's broke.
Sorry for the rambling there... I spent 20 years on the systems side of things before moving into development, so there are a few sore spots.
Anyway, based on what I have seen, at least for the current generation (Visual Studio 2010 and the various other tools around that), Silverlight will be around for at least another 5 years or so (which is the typical life-cycle of an MS product).
And, as long as the plug-in works with current browsers (whatever they may be), anyone can still write the code (with the right tools) and still have the plug-in for download. So really, it's more of a browser issue than an OS issue.
So, with all that, come 5 years, who knows.