Personally, I believe that XAML has a long healthy life ahead. Almost everyone I’ve spoken with who programs W8 in both XAML and HTML says that they are more productive in XAML.
It is the shared language not only of Silverlight and WPF but of Windows 8 and Windows Phone. These are serious commitments by Microsoft.
But I have a strong bias: I’ve invested six years in developing expertise in XAML, my job is as a XAML/W8 Evangelist for Telerik and I have two books coming from Apress on Programming Windows 8 with XAML.
At the moment I’m not putting time into learning the JS side of the house; but I’m keeping my eyes open. What about you?