I'd like to emphasize that I do not encourage users to make use of this feature! I'm trying to gauge how disruptive a deprecation will be (if at all) to figure out whether I can rip this band-aid off quickly or if I need to provide a more gradual transition.
Getting rid of inheritance will greatly simplify Decker's implementation and help pave the way for better capabilities in the future. Instead of filling out "template cards", I think it would be much more powerful and flexible to have a mechanism for making "custom widgets" that can be re-used throughout a deck and imported/exported like fonts and modules.