"The only stalwarts here appear to be Backbone (other than its default routing feature, it works on IE6) and Knockout (which for now at least insists on total IE6 support)." IE6 support is a must-have for me (corporate clients).
Is the ie6 support accidental or do the devs sacrifice maintainability, readability or performance to specifically support it? It seems to me that since it is so small, and doesn't really do much, they'd have to go out of their way not to support ie6.
Oh, how I wish! Try searching the Knockout source code for "IE" and see how many special-case rendering and event bugs it has to work around. For sure, getting 90% IE6/7 support might be quite easy, but that last 10% is a real pain and that's the value of using a mature framework where someone else has already debugged the heck out of it.