Very true, but with the caveat that you have now increased your motor, prop, and ESC budget by 50%. And of course this will impact your battery life, as coaxial arrangements trade away some efficiency for this redundancy.
Then just make it a hex copter and do away with the coaxial arrangement. You still have 6 motors, 3 per rotation direction. With good software you could still fly with the loss of one motor per rotation direction with practically no control degradation. You could lose 2 on a single rotation direction and still likely control it enough to land.
