I’m still working out my whole response to the Microsoft versioning idea (and feeling sad as the WaSP eats itself).
But there’s one practical problem with the suggestion that pages without the MMM™ (Magic Microsoft Metatag™) become ossified under IE7 for all time. IE7 (and its predecessors) makes some in-page links entirely inaccessible to keyboard users. Yes—completely unreachable to blind people and those with motor impairments.
Read Gez Lemon’s Keyboard Navigation and Internet Explorer article for a full investigation of the problem, but in a nutshell:
…when using the keyboard to navigate to an in-page link, pressing the tab key once the target has received focus causes IE to continue navigating from the start of the document (or more accurately, the closest ancestor with the hasLayout property set to true).
If you have IE7, you can check out my IE7 keyboard navigation test page. Activating the first link should put focus in the last paragraph; a further tab should take you to the very last link on the page. But IE just takes you back to the top, making that link impossible to reach.
There are code workarounds—but if a site-owner doesn’t know to add the MMM™ to the top of their pages, they’re unlikely to know or care abut those workarounds.
So freezing the web to IE7 levels consigns much of it to perpetual inaccessibility.