Quoting
Keith.Williams
Why don't we just remove the nozzle and get rid of the rearward acting force?
The above results suggest that this will give us more resultant thrust.
Simply because the nozzle gives you much more thrust. The purpose of the nozzle is to accelerate the exhaust air and ideally expand the exhaust pressure to ambient. Higher jet velocity means more thrust. Nozzles become much more important with supersonic jet engines and reheated engines. Hence the use of variable geometry convergent-divergent nozzles.
The same goes for the nozzle on a rocket motor. It's not just there to steer the gas stream and vector thrust.