Joshua Gans outlines the counterintuitive argument at the Harvard Business Review. The underlying assumption--that the toy is being used as "bait," he says--is sound.
But let's imagine for a moment that McDonald's sold Happy Meals with toys while another chain did not. If the meals were equivalent in the eyes of the children--say, each had 10 units of the "bad stuff"--then McDonald's would win. So the other chain, in order to compete, would have to either include a toy or make the meals more attractive by including, say, 15 units of the bad stuff: more fat disguised as potatoes or what have you.His point is that McDonald's might try to put more fruit in Happy Meals to be allowed the toy. But it's also possible it might try to compensate for the lack of toy by adding more fries, or perhaps a candy or cupcake. Furthermore, should it go the apple slices route, "for parents, the new, healthier Happy Meal has become a harder sell relative to other options with poorer nutritional value."
Technically the law seems to ban toys with fast food in general unless the food meets certain guidelines, which include sodium caps and a serving of fruits or vegetables. So any competitor who wanted to offer toys would have to meet the same health criteria as McDonald's. Nevertheless, it's an interesting scenario to contemplate.