I recently got to know about washes using watercolors diluted with Future. With such washes, airbrushed postshading seems unnecessary (I got back to modeling after a long break, so forgive me if this question is dumb). Am I missing something?

Washes are meant to get into panel lines and recesses, accentuating the depth of detail on the model. Post shading is a visual technique to darken either the edges or center of panels. They produce different results. The best thing to do is take an old model and do both side by side, see which you like best, or use them together. They are not mutually exclusive techniques.