The entire point of the suggestion is to break compatibility with IE6 so that the rest of the world can stop having to code against it. Breaking compatibility with IE6 has freaking enormous switching costs for some users.
(Let me hum a few bars: you use a $3 million CRM which only supports IE6, and the company which bought the company which made it has since folded. This is hypothetical, but not very hypothetical, if you catch my drift. A forced free upgrade to IE9 would create an organization-wide emergency for that customer, instantly, and it would be a cold day in Hell before they every do business with Microsoft again.)
The entire point of my suggestion is that a newer browser doesn't need to break compatibility with older browsers to allow people to stop supporting them. Backwards compatibility makes it possible for Microsoft to do a forced upgrade without screwing anyone over. Once a significant majority people are using IE9, it doesn't matter if they're still relying on its backwards compatibility for existing websites, new websites can just target IE9's native rendering engine and ignore older versions of IE.
I didn't mention IE6 backwards compatibility, because no one running IE6 is going to upgrade directly to IE9; they don't run on the same OS. Even if IE9 was made a mandatory or automatic upgrade, people relying on IE6 wouldn't have to worry. IE6 would have to be dealt with differently than IE7&8, which is fine, it'll get to insignificant market share soon enough on its own.
The currently existing alternative is just to install Chrome or Firefox for the current web and leave IE6 in place for legacy internal applications. But then browsing the web isn’t seen as a useful part of people’s jobs in most companies.
> $3 million CRM which only supports IE6, and the company which bought the company which made it has since folded.
You can make it entirely realistic by saying the company that made the CRM now has a version that supports Firefox and IE7-8 but it costs $5M to upgrade the license. I've had so many of those cases at my current job.
(Let me hum a few bars: you use a $3 million CRM which only supports IE6, and the company which bought the company which made it has since folded. This is hypothetical, but not very hypothetical, if you catch my drift. A forced free upgrade to IE9 would create an organization-wide emergency for that customer, instantly, and it would be a cold day in Hell before they every do business with Microsoft again.)