Agreed on all points, and to add another: Webkit is in use by more than one competitor. There are the occasional shootouts between Chrome and Safari to be obvious, and so long as both wish to be preferred by their users, they'll both continue to innovate or, at the very least, prevent the stagnation that IE6 became.
Browser shells don't count. We're talking rendering engine here. Trident (IE6's engine) was fully controlled by Microsoft and until Chrome Frame came out (and promptly not adopted at all) there was no way to change the rendering engine in Internet Explorer.
As r00fus mentioned, I think that's giving Maxthon a little more credit than deserved. The 'core' of IE6 was still IE6, and adding a bunch of fluff around it didn't change that. As such, the IE6 engine, the thing that mattered, went untouched.
That isn't to say that Maxthon wasn't awesome, it was. But it also wasn't nearly as pervasive as even Safari is really.
