This is Richard Monson-Haefel of Curl Inc. We also offer a "presentation layer abstraction". In fact, Curl came out in 1998 so it predates Adobe flex and silverlight. We've been offering a platform that provides both RIA and desktop runtime since 2004.
I think you missed a few things in your analysis. First, when Java applets came into vogue (I was there writing them) consistency among implementations of the JVM was horrible across browsers. This is why its failed. The same was true of DHTML which is today called Ajax.
The pore portability story around Java was solved for the most part when Sun took charge and offered their own browser plug-ins, rather than trusting vendors to implement the JVM properly. Sadly (for Sun), however, Java already had a stink about that they just can't shake on the desktop.
Java was/is also a bit of pig in terms of performance on the desktop - that too has stymied its growth as "presentation abstraction"
The same cannot be said for Silverlight, Adobe Flex/AIR or Curl. These technology do not have a lingering stink about them - they are, from the perspective of developers, fresh. In addition, they also leverage existing developers skills in web presentation (Adobe) and .NET knowledge (Silverlight). Curl is the least familiar of the RIA platforms but its been in production use in Asia for five years, its much faster, and more secure. So Curl has its own strengths.
Anyway, you need to look at the whole picture. The truth is that in the future desktop abstractions will prevail. Of course, since my company offers such a thing I'm biased, but I'm also right.
All the best,
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