Push-pull zoom lenses were invented for manual focus systems. In that application, they made perfect sense, since they allowed the photographer to both zoom and focus without repositioning his/her hand on the lens. I owned three in Canon FD, including the Vivitar 35-85 f/2.8. I also used extensively the Canon 80-200 f/4 FD L zoom lens which, as it happens, was a 2-ring zoom.
Guess what! With AF systems, most of us let the camera do the focusing most of the time. I currently own the 100-400 and a 2-ring 70-200 Sigma EX, plus three shorter zooms, all 2-ring. The 70-200 has been a workhorse for 12 years. The 100-400 had to be sent to Canon for repair after it stiffened up and ultimately froze at 400mm. I've never had problems with dust though. Count me among the people who badly wants the next-gen 100-400 to be 2-ring, with the zoom ring behind focus ring.