Why doesn't Baymax get on the ship and fire his fist away from the exit, so that everyone, him included, is sent towards that exit and saved? Why does he chose to fire his fist into the ship, thus sending it towards the exit and him away from it?
A poster on Reddit explains:
The only reason they made it out in time (remember the exit was closing quickly) was because that arm kept pushing them, accelerating them with it's propulsion thrusters.
In your alternative plan, A) they're heavier by one armor clad robot, thus slower to accelerate with Abby given force and B) their only push would be from the initial ejection of the arm. All of that propulsive force from the thrusters would do absolutely nothing for them. As it would be pushing them while accelerating in the opposite direction. They'd never make it out in time.