One of our Google Summer of Code students implemented CardView last summer, actually. Merging it was slowed down because several other changes were developed in parallel, and that was the logical one to go in last. I’m working on finalizing the pull request now.
Regarding versions, you’d be surprised how much usage there is for fairly old versions of Android.
You can make a pretty good case that the very oldest versions we support are in such sparse use that it isn’t worth the amount of work it takes us to support them. There are still a few years worth of Android versions in active use.
If you really want a tool that is tuned for the most cutting-edge part of the world, you’re probably better off with Kodular or another commercial project using this codebase.
Worth noting: We aren’t holding ourselves back to Android features only available in API7. We are making as many features available that far back as we can and ensuring that applications developed in our Designer won’t crash on versions that old.