Most people who can't or don't know how to upgrade away from IE7-10 won't be able to manage installing a plugin either. That aside, I don't consider a beta plugin to be an acceptable answer to this question in any context.
Have you actually used or tried to deploy ChromeFrame? Its plenty ready for prime time, and features:
1. A 60sec installation
2. No browser restart required
3. No admin rights necessary
4. IE6+
5. Autoupdate just like regular chrome
Even the most technophobic user can follow 2 links to be able to use an app they really want to use.
And remember this is all about applications, not your average brochure site.
ChromeFrame is great, but it is probably against the usage policies of a lot of companies. Even if it isn't, most people are fairly wary of installing something on their company machine. You'd need the IT department on side to get broad deployment.
I'll admit, I haven't. That feature list is mighty impressive and all kinds of commendable.
That said, it still fundamentally is requiring people to install something on their machine. This is a difficult thing to do with the uneducated and often outright forbidden in the corporate world, so the two key demographics here are also the two least likely to use ChromeFrame.