If you want something with better deployment stats, look up Adobe Flex. It's more mature than Silverlight but it's currently the leading RIA SDK and doesn't suffer from the low availability problem of SL. If you're seriously considering switching to AJAX/JavaScript/DHTML hacks--don't. This is like taking a step backwards in terms of technology. You're missing many of the advantages in addition to those you've mentioned of using a virtual machine based approach to rich client development:
O&M costs: Most "engineers" don't give a rats A about this because they are development-centric, but the fact is that O&M is by far the leading cost of running IT systems. With a VM approach like Flex (or Silverlight, or JavaFX) you don't have to worry about updating libraries, testing, applying fixes, and so on when a new browser comes out.
Performance: Flash (Flex) is JIT compiled, (wouldn't be surprised if SL is too), and Java is as well. Interpreted JavaScript dependent libraries such as GWT simply cannot hold a candle to the performance you get out of a compiled VM client--it's not even a close comparison. (Just look pay attention to how slow Gmail typically loads--a very simple AJAX-enabled script.)
Consistency/Reliability: Everyone knows that when you write anything using JavaScript or HTML, it has to be tested in every browser (including some older versions if you really want to be safe). No such problem when you're targeting a VM platform. With AJAX: worry whenever a new browser or version of a browser is released. If there's problems, it'll create work for you.
Job Market: While AJAX/JavaScript/DHTML is still very common (depending on the area you look), there's dozens of "frameworks" out there. An expert in DOJO is not an expert in GWT. An expert in GWT is not an expert in jQuery. Etc. If an employer is looking for a Flex developer (which is becoming quite common in job listings I've seen), they actually mean Flex and not some random framework built using Flex. The point is: VM-based job markets like Flex or Silverlight are not fragmented like the JavaScript/DHTML realm is.
Richer components and a better user experience: This is one of Adobe's big selling points, but it's true. While some of the stuff that has been done purely with browser scripting (JavaScript) is impressive, Flex or Silverlight components are better. It's also easy to create your own custom components when you don't need to write (and maintain!) cross-browser JavaScript magic.
Flex specific, but worthy of mention--AMF: This is the Flex Data Services binary protocol which can be used to talk to servers. Benchmarks (ask Google) have demonstrated this protocol to be much faster and more bandwidth efficient than protocols used by AJAX frameworks.
It's a sensitive topic for many. Most AJAX/DHTML/JavaScript 'engineers' or whatever they call themselves will howl and throw feces at the mere suggestion that this 1990's technology they have spent years becoming an expert in is outdated and flawed (quite frankly they have a lot to lose). But based on pure technical merit, even HTML5 is a counter productive waste of time compared to any mature VM based solution. I'm sure some will show up shortly to defend their cult of denial.
Imagine what would have been possible had Google, instead of throwing who knows how much money at JS and Google Chrome (not that Chrome is a bad thing), had instead created a free open source plugin competitor to Flash? Gmail wouldn't be doing email: it would probably be sending webcam-recorded video mail or live feeds in addition to that, and who knows what else would have been possible.
It would be nice if the browser finally evolved into an application delivery mechanism for standardized VM-platforms such as Flash, Silverlight, JavaFX, or whatever OSS devs can come up with. Time to move out of the 20th century yet?