Power users tend to build muscle memory even for UIs. I tend to move my mouse to a particular location on the screen for the delete button for example.
With a product that has so many existing users, I think GMail should pay attention to these details as well when planning a new design. So, should other companies with similar products. An epic failure in the same lines is removal of start button by Microsoft. Of course a button position is not as critical; but the point is to have this criterion included in the design process.