Very long post ...
I agree in that, I think, the whole point is to get you over to one side or the other. If you're using XAML you'll most likely be using Visual Studio to create and design the Markup. And you'll already be using some .NET technology and tools, and they would hope it will lead you to continue in that way. Kind of like a gateway drug
I think, it will get traction because every dime a dozen .NET developer is already familiar with XAML that will bring it to their web applications and would naturally prefer using it over other similar technologies like Flash. In a sense it gained traction before it was even released.
Last year I started experimenting with SVG (and SMIL animations), it's a W3C standard, Adobe was strongly backing it, FireFox and Mozilla have pretty complete implementation of the standard. It's a well known standard, cross browser, cross platform, several implementations, sounded great. And then one day, out of the blue, Adobe pulled out of SVG after they figured it would get in the way of people using their newly acquired MacroMedia Flash products.
Their ASV plugin (which is the richest, and most popular SVG implementation and the only way to get SVG on IE) support will be dropped in a few short month (they even went further saying some features are known not to work on Vista) and a year later the plugin will not be available for download and they would go after anyone distributing their plugin. Oh, but they did say we could go over to Flex for similar and even better functionality
So, I feel, standards isn't the only thing to consider. XAML is very powerful, cross browser, cross platform, I'm pretty confident it will hang around for a long time and have very strong support and a big community. And, hey, I wouldn't mind going into .NET someday. I think, WPF/E should certainly be considered as a UI and RIA option.
In regards to OpenXML in short I'll just say that I think it's going to be the new PDF. Because besides everything else it's not only easy to generate documents but it's also easy to consume them. That invoice document doesn't only look great but it's also easy to get the data out of it. One day companies and government are going to require it. And then again maybe not
Another well known and related standard to OpenXML is XSL-FO for creating document, which lacks support.