XP Mode is limited to Professional and Ultimate/Enterprise versions of Windows 7. These aren't going to be the versions that consumers (and small businesses) are going to get when they buy a computer from Best Buy or Dell. When those people replace their aging computers, they'll get Windows 7 with IE8. And unlike when XP first came out, most new computers are configured to automatically download and install critical Windows updates (which includes new versions of IE)--so IE8 won't be the albatross is that IE6 has become. Most Windows 7 users will be upgraded to IE9 without even thinking about it. (In this sense, IE8 may be the shortest-lived version of IE to date.)
That means this is really only a concern for corporate users, where policies are dictated by budgets and IT departments, not a lack of basic computing knowledge.
One reason that many such companies force their employees to use IE6 is because long ago they developed custom intranets or other internal web-based applications targeting IE6 and they're not willing to spend the money to upgrade or replace them. And because no other browser comes packaged as an MSI (meaning they can't be centrally managed and deployed), their IT departments won't let them install Firefox, Opera, Safari, or Chrome. That only leaves IE--specifically, IE6. (Want to see an uptick in Firefox usage? Lobby to have Mozilla release Firefox as an MSI.)
Now, because of XP Mode, corporations can upgrade their users to Windows 7 without fear that they'll have to spend money upgrading or replacing their intranets/apps. And because Windows 7 comes with IE8 (which can't be downgraded), those companies are going to have to put policies in place to secure it to their liking. Their employees will then be able to use IE8 for the web at large and IE6 for their intranets/apps.
For that reason, many companies will be more likely to upgrade to Windows 7 sooner than they would have done so if XP Mode wasn't available.
If some corporate IT department wants to upgrade to Windows 7 but still reinstall IE6 over it for some crufty legacy app compatibility, I think they'll probably have much bigger problems that would make the OS upgrade pointless altogether.
Granted, Windows 7 currently ships with IE8, so we'll still have the same problem for a while.
It's a sad situation really. It's easier to install IE6 than rewrite the apps to work with another browser.