This article on Infoworld pits these proprietary Rich Interface Apps against the HTML5 canvas element. HTML 5 might "eat these vendors' lunches", "invade their turf" and "threaten" to render them irrelevant. While there are conflicts of interest, I see a huge opportunity here.
My idea? The canvas element is just a tag, with a specification for the functionality it provides. How it is implemented is up to each browser. Why does this have to be an either/or situation? How about some competition? Why can't Microsoft's Silverlight be their <canvas>? As long as it provides the W3C canvas API, in addition to Microsoft's own functionality, I see no issues. JavaFX and Flash could similarly implement the canvas functionality, and offer an alternative to a browser's built-in canvas. Or in the case of IE, Silverlight could be the canvas implementation.
In this way, the whole issue of HTML5 vs the proprietry RIAs goes away, replaced by something resembling the browser market currently. A user could choose which technology powered their canvas - for example, in Firefox on Windows, the options could be: