Except that flex does NOT have to be one dimensional if you get flex-wrap involved and understand flex-grow and flex-shrink.

To the point that if you don't mind the footer going under the menu section in your example, you can do that in flex too. Just set header and footer to flex:0 0 auto; width:100%; and main/right to flex:1 0 auto; with right set to an elastic or % width and main set to auto. If flex wrap is on the container, it'll work.