This is a-historical by my memory - the entire reason Gmail became so successful was that its use of "fancy ajax interactions" made it shockingly nicer to use than any of its competitors at the time.
I think if one of your main selling points is interactivity afforded by javascript, then it is unlikely to have much return on investment to accommodate users who don't want that. But for sure people should be thinking harder about what their selling points really are - the market for "it's better because of ajax!" is probably tapped out.
