Although the PVA was able to win battles in early '51 using primarily light infantry, the UN forces didn't suffer heavy casualties because of the ineffectiveness of the weapon systems used by the PVA. And because most of the PVA soldiers were on foot, they couldn't even catch up to the escaping UN troops. So it would be illogical to expect the war would end there even if Mao unilaterally decided to stop. Since the UN had preserved its strength, it would only be logical to expect them to come back with a vengeance, which they did. The PVA generals understood this and did everything they could to quickly arm their troops with heavy weaponry, which played a critical role in the final outcome of the war.
In fact, this example shows precisely the weakness of using light infantry. They may win a battle or two through surprise attacks and quick maneuvers. However, they cannot inflict heavy casualties on their enemy. Eventually, their enemy will be able to regroup and come back with even more powerful weapons and technology. When that happens, the light infantry will be in an even worst situation. remember the light infantry has to depend on their surprise and number. Because of the ineffectiveness of their weapons, they have to sacrifice their soldiers in any attack. The longer they stay on a war, the more depleted they would become the less effective they would be. It won't take long until they would completely lose all their advantages.
Another critical point is the supply. with the light infantry, one would not expect them to have any advanced means to maintain a supply line. They have to keep moving to avoid being detected. So their supply line will be almost non-existent. How would they keep their soldiers fed, keep them warm and make sure they have enough ammunition? The more they maneuver, the farther they will be from any possible supply route and the more likely they will starve. And the more they move around, the quicker they will starve and lose their morale.