Galileo’s argument suggests that an unbalanced (external) force is required to change the motion of the marble, but no net force is needed to sustain its uniform motion.
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If the right-side plane is made horizontal, the marble would continue to travel forever, trying to reach the same height that it was released from.
The marble will roll down the slope and go up on the opposite side to the same height from which it was released.
It is difficult to achieve a zero unbalanced force in practical situations due to the presence of frictional force acting opposite to the direction of motion.
Galileo’s thought experiment suggests that no net force is needed to sustain the uniform motion of the marble; an unbalanced (external) force is only required to change its motion.