It's something I'd like to give for a consideration. After latest work on redoing project navigation as a service so that other apps can consume it, I can also render footer navigation from project navigation data as a part of our project template. So, I think that if user adds project navigation to his/her site then footer navigation should be generated automatically from its data.

Since some people don't use footer navigation, we could have a checkbox in project navigation dialog asking if the person wants a footer nav being rendered as well or not. However, I'd see it as checked by default because I think those project that don't use footer navigation, do that because it needs double work to maintain it in sync with project navigation.