Chairman Mao, when meeting the Japanese physicist S. Sakata (who was one of the poineers for the quark model), stated: “Elementary particles can’t be elementary”. Sheldon Glashow appreciated this philosophy and hence proposed that we should name the sub-structural particles under quarks “Maon”. Do you think that “Maons” must exist or not, based on physics arguments rather than your belief?
There is really no physics law that hinders Maons from existing. Thus, we can't really say with certainty that it does not exist. After all, ancient Greeks (Leucippus and Democritus) thought atoms are indivisible already. They were however proven wrong when our technology allowed us to probe dimensions than is in the order of the size of atoms. In order for us to probe the constituents of quarks should they exist we have to go to an energy regime we can't access yet. What makes it more complicated too, if you apply energy (by smashing another particle onto it) to a quark you will end up generating more particles.
If we don't have a way to test its existence or non-existence, then its value as an idea is not so important yet in a scientific point of view.