RIA adoption rates in the corporate intranet world seem higher than that of the Internet because the cost-effectiveness and efficiency is being recognized there. (A growing number of job postings asking for Flex experience in my IT-heavy region demonstrates this.) Why not also on the Internet?
Reasons you get from people who favor hacked-together JavaScript/AJAX/DHTML solutions just don't hold up to scrutiny. For example you hear RIA 'downsides' like:
Vendor lock-in... Using supposedly "standard" web techs you are still locked in to whatever JS framework tool you choose. What's the difference? It's even worse because the heavy competition between the multitude of "AJAX frameworks" means your FW of choice is more likely to be abandoned some day.
Search engine indexing... This is an issue for AJAX websites (anything with very dynamic content) too, not just RIAs. And for static content, Google has been indexing SWF (Flex) content for some time now. Anyway, this is a problem for web-SITES, not web applications: two entirely different things. And deep-linking is not a problem for a Flash app, even basic research can dispel that myth.
Performance/interoperability... Flash/Flex is JIT compiled. (If Silverlight isn't, I'm sure it will be eventually.) JavaScript is interpreted. Poor performance? Really? Flex runs in a platform independent virtual machine. Compared to the 'platform incompatibility' problem within browsers for AJAX scripts, saying a VM approach is not as cross platform is completely backwards thinking which completely ignores the browser wars and compatibility problems.
Accessibility... Any large website needs to think about this. Contrary to what many believe, Flash has accessibility capabilities--it's not vastly harder like many claim, just different.
Player 'upgrade nightmares'... Not an issue for a consistent backwards compatible platform like Flash. Actually the non-standard, inconsistent DOM and DHTML browser incompatibility problems common in AJAX applications seem like the real nightmare in comparison.
To make it worse, the list of "downsides" (such as O&M maintainability issues in response to new versions of browsers which break AJAX scripts) is much longer than any list of issues with virtual machine application deployment. Why do so many hesitate to adopt 21st century innovation and abandon the 1990's legacy JavaScript era? Is it just job security and learning fatigue from a rapidly evolving IT environment? What is the difference between the software and hardware world where they are adopting virtualization technologies so readily?