Earlier quoted context omitted.
For a few decades people accessed email on text based terminals. There's plenty of applications with more complex UIs than email...
I have other applications with more complicated UI's also relying on HTML and AJAX and they work just fine. What kind of web applications do you have that would require Silverlight or something similar?
I'm sure the Gmail team could do a bunch of hacks to fix this, but that's the problem. You have to hack-up everything to make HTML work well as an application platform.