This is, I think, one of the difficulties of the game that could be improved. Once you get a nice evolving building and put it in place, if you raise it to a certain level it becomes pretty much permanent. Unfortunately, it doesn't grow much even when you upgrade it (as vs evolving it) and thus, unless it's a evolved all the way, it slowly becomes less and less useful. I have 2 mermaids, one evolved to lvl 10, the other to lvl 5. The lvl 5 one is borderline usable, but I hate to get rid of it if they are going to ever offer the ability to evolve it to lvl 10. If I could park it in storage with it's current level I would. And then I'd take it out when they offered the artifacts to evolve it.
Thus, I agree that having the ability to park an evolving building would be a nice thing and make things a bit more fun as you can then get excited when they finally offer the artifacts to evolve that building more rather than kick yourself because you got rid of it two months ago.
AJ