I think there is more than enough circumstantial evidence to suggest the US vs Microsoft settlement played a significant part in the stagnation of Internet Explorer. Internet Explorer 6 was released on August 27, 2001 [1]. The anti-trust case was settled on November 2, 2001 [2]. After IE 6, Internet Explorer effectively went into hibernation for five years.
Was there correlation? I don't know for certain--I wasn't there. But it seems likely that work stopped on IE in large part because of the settlement. For this reason, I would remind any web developer who complains about IE 6 to reserve a bit of their vitriol for the United States government for meddling in this marketplace.
After a while, Microsoft started testing the browser water again, slowly at first. I now think IE 10 is as good a browser as Firefox or Chrome, perhaps in some different ways. I remain a Firefox fan, but I am glad that Microsoft is back in the browser game because I was already starting to feel nervous about Webkit domination in the past 1-2 years.
[1] http://en.wikipedia.org/wiki/Internet_Explorer_6
[2] http://en.wikipedia.org/wiki/United_States_v._Microsoft_Corp...