Fair point. You can look back at a number of features added to the HTML spec, and note that they were first implemented off-the-cuff in IE6. XHR, contentEditable, to name two I can think of straight away.
We can give them as much shit as we like for letting the ecosystem stagnate while they had control, which Firefox thankfully set out to fix, but would we have had AJAX if there wasn't an existing XHR implementation, or anything else implemented outside of the standard?
It's so hard to say: maybe it'd have taken a while longer. On the other hand, maybe the alternative solution would have been simpler, allowing later developments to progress more quickly. It certainly took years to clean up IE-specific html mess. Who can know if it's a net positive...
We can give them as much shit as we like for letting the ecosystem stagnate while they had control, which Firefox thankfully set out to fix, but would we have had AJAX if there wasn't an existing XHR implementation, or anything else implemented outside of the standard?