IE9 does have compatibility modes for IE7 and IE8 already (somewhere on the official IE blog there's a flow chart describing how IE decides what rendering mode to use for a given page; it's terrifying)
My understanding is that IE's entire user-base consists of update-averse users these days - everybody who can stomach the idea of changing browsers is already using Chrome or Firefox.
Not quite. IE8 is making huge gains in market share, but almost entirely at the expense of IE6 and IE7. The overall IE marketshare is holding somewhat steady at 60%. I think what you're seeing is Windows users are perfectly happy to take an upgrade that Microsoft offers, but either afraid to branch out to a non-MS browser or else just don't know how to do so. Windows 7 shipping with IE8 has a lot to do with it, too, as people buy new laptops, etc, and never touch the browser it comes with.
My understanding is that IE's entire user-base consists of update-averse users these days - everybody who can stomach the idea of changing browsers is already using Chrome or Firefox.