Effectively we do already support cameras going back not just one version, but several versions. You may be surprised to hear that you can even use ACR 2.4 to read, say, Nikon D90 images, or Canon EOS 50D images. This is possible via the DNG workflow (i.e., convert the file to DNG and then open it in any DNG reader, such as ACR 2.4, ACR 3.3, ACR 4.6, Silkypix, etc.).
I honestly believe that going back and modifying the old code from earlier versions to directly support new cameras would end up hurting customers more than helping them. It sounds good at first, but there's a tradeoff to everything. Ultimately I believe it means that it would
(1) take us much longer to add new camera support in general, so instead of waiting anywhere from zero days to a few weeks for new camera support (the current situation, typically, unless we get tossed a brand new raw file format ...) you might be waiting from weeks to months,
(2) take us much longer to add new features (things like local corrections, the new color profiles, and a few big things we've got planned that photographers have been asking us for a lot), or
(3) increase the cost of the product,
or some combination of all three.
In my view, the fundamental issue is that going back to add direct camera code support (i.e., non-DNG workflow) to the previous versions doesn't actually improve your images at all. There's no quality difference in CR between converting, say, a Canon CR2 file and its converted DNG. Yes, I understand there's a convenience/workflow difference, and some folks frown upon DNG. But ultimately it's the image result that counts, and there's no visual or quality benefit from investing the enormous resources required to do so. In contrast, there IS a big quality/workflow benefit from improving existing features and developing new features, things that will actually make your images appear better. I hope we can all agree that the CR team's time & resources are better spent on that.
(And since I've already written on this, I might as well say that we smile every time we see a new camera released that supports in-camera DNG, such as the new Casio EX-FH20, because that means we don't have to spend time writing code to support that camera, so instead we use that saved time to work on new features.)