Piggy cannot devolve into savagery. Because he cannot change, as symbolized by the fact that his hair will not grow, he cannot adapt. He cannot adapt to the savagery of Jack's world. Civilization protects the weakest. On the island the law of the jungle where the strongest survives overrules the laws of civilization which protect the weakest of its members. That includes Piggy. Piggy is not physically strong. He has asthma; his is overweight. These attributes make him particularly vulnerable in a warrior society, which would have no use for someone like Piggy.
But Piggy would be able to be a very productive member of a civilized society. He can think. He is logical. As Ralph notes, "Piggy can go inside that fat head of his and reason." Piggy can contribute much to a world in which there are rules, laws, and order. But in a world where physical strength is prized, Piggy is at a loss. He cannot function in a warrior society, a Spartan society, a primitive society.
With Ralph's leadership, Piggy's particular abilities are noted and appreciated. With Jack's leadership, they are irrelevant.
If you are put down in an uncivilized environment, I think that you have two ways to survive. First, you can, yourself, be strong physically and mentally. Second, you can induce people who are strong to be your friends/allies -- you can convince them that you are in some way valuable to them.
Piggy does not fit either of these categories. Of the characters that we are really introduced to, he is by far the least strong. He has his asthma, he is fat, he seems like he is emotionally pretty fragile. With these traits, he surely will not thrive on his own.
Secondly, he does not have a personality that will allow him to seem useful to those people who do have the power. He has the kind of weak personality which (along with his physical shortcomings) seems to invite abuse and hatred from strong people.
So Piggy is not strong enough to survive and he lacks the attributes that would allow him to become part of the tribe that gathers around a truly strong savage person.