There is a fundamental problem with RIAs; the open web is a much better platform to build applications on. Every day browsers get better and the things you can do with HTML, JavaScript and CSS are for 90% of the applications a good solution. The other 10% of the applications a developer has a choice where RIA might make sense but 5% of those applications are probably better off being a desktop applications.

This leaves 5% of the applications which would benefit from an RIA, these apps are typically internal data entry or dashboard applications which don't get a lot of press.

Over the last 6 years I have worked in the RIA space and it really comes down to a simple question. "Would you want Google to rewrite the Google Analytics or Gmail in Flex/JavaFX/Silverlight?"

For me the answer is NO!. The best applications today are open web with Ajax / Flash enhancements to improve users experience.

Google Analytics uses Flash and Ajax in this exact way; Flash for the charts and the time span widgets but the rest of the application is HTML. Ajax is used for doing sorting and paging of data.

This is why there will never be a killer RIA application.