Is this what-if really worth thinking about? If and when that time comes whoever is using IE can just switch to one of the half-dozen other browsers on their platform.
When IE6 stagnated, it had a market share of over 95%. So if a web developer wanted to take advantage of new HTML/CSS features, only 5% of their users would notice, and so it wasn't really worth the cost. This meant that all websites had to work in IE6, so there was less incentive to switch browsers.
Now IE market share is closer to 40%, so whilst they're still the market leader, they no longer have a majority. Even if Microsoft ceased development on IE9 now, web developers could still use new HTML/CSS features and reach 60% of their audience. This would mean more websites using new features, which would result in a faster switch from IE.
If you have a monopoly, you can afford to sit back and allow technology to stagnate. If you don't, and your competitors are constantly developing new features, you'll lose market share rapidly unless you keep up.
your competitors are constantly developing new features, you'll lose market share rapidly unless you keep up
This, in a nutshell, is why IE9 is not oing to be "as good as it gets". To be sympathetic to IE9, it's a good start. But only a good start. The people inside MS who plan IE must know that there's more work to do in order to keep up with the competition, let alone speed up and overtake them. MS is used to running marathons when they have to, not betting it all on one version.
That isn't the point. The point is that it will take a very, very long time until the (outdated) IE community is small enough for developers to ignore.
And now things have changed - it will take a very, very long time, or more likely never happen, for the Firefox, Opera, Chrome, iPad and android community to be small enough again for MS to ignore. So IE has to progress.
