IE6 market share is below 7% today and falling every month. There's nothing to explain.
The Netscape/Sun idea of abstracting away the OS so that you could do useful work on 'any' computer was smart, and was the most significant threat to Microsoft's hegemony in the 1990s, which is why MS were rather obliged to pull all those dirty tricks, which got them in hot water with the antitrust boys, and which still echoes in the EU-enforced browser chooser.
Microsoft must be peeing their pants at the prospect of losing not just one cash cow (MS Office) but both (Windows). Considering that MS loses money on almost every other aspect of their business, it's no wonder that they are now banking on Bing to compete directly with google at *their* game, and it's no wonder that there are competing factions within the mastadon.
Microsoft has simply grown too big, with various departments producing software which compete with (and are incompatible with) the products of their other departments. We're seeing in-fighting and the legacy of inflexible business models, which indicate that they got addicted to the milk of their own cash cows and forgot that there are other nourishing drinks - with considerably less fat - which might be made available for free.
MS is moving too slowly to keep up with the rest of the industry, which is sad, because I was just beginning to like them. Maybe they would have been more viable today if the DOJ had split them in two.
As for using HTML5 for making the GUI of 'real' (i.e. not web-based) apps. I have to ask 'why not'? XAML and HTML5 are both markup languages, why should one be better than the other? The processor intensive work (e.g. video decompression) should/could be handled by appropriate hardware anyway, and we haven't yet seen the impact of GPUs on non-graphical number-crunching tasks like real-time audio filtering. (This positions Apple in a very advantageous spot).
HTML5 can be optimised with the right tools and be no worse than Qt or any of the other presentation frameworks.