There's a couple issues with this, not least of which is the fact that you will always get some crazies who take advantage of that to start browsing the web in their favourite antiquated Windows OS with absolutely no protection.
That assumes as well that Microsoft is willing to get these old OS's to work, even in a hybrid boot with modern hardware. And if we're talking about using one OS that boots into compatibility modes, I think that opens up even more holes in the OS, as you all of a sudden have legacy software present and possibly accessible from the primary OS.
At this point, I think Microsoft's best bet is to offer downloadable Virtual PC images with 3.1/95/98/etc. and spend some time getting 3D support to work properly from within those OS's, and make a reasonably clean break from there. You would still need to support the software from the last release or two of the OS, but it would allow for more forward thinking.
We're getting to the point where we have enough PC horsepower to handle this situation, at least within reason; and actually, the Windows 8 DP has support for mounting .VHDs built in, which makes me wonder if that isn't the way they're going.
.... Already typed this all out when The E posted, so to heck with it I'm posting it. ninja'd though.