While I think gluten intolerance and allergies are overblown to some extent, your argument for their non-existence makes literally no sense. Many, perhaps the majority, of human beings are allergic to some sort of pollen, i.e. they get an immune response merely from going outside. Or they're allergic to pet dander, and the domestication of those animals is older than the domestication of wheat. Celiac's is life threatening but many people get a relatively minor immune (allergy) or gastro-intestinal (intolerance) response when they eat gluten. That isn't an illusion.
Keep in mind vast parts of the world up until very recently included very little if any wheat in the diets of the non-wealthy. Even in the US, it's only been recently that wheat has begun to replace corn as the staple diet crop, newer strains of wheat have higher concentrations of gluten, and extra gluten is added to many bakery products to shorten leavening time. There's a number of reasons that prevalence of gluten-related disorders, including Celiac's, may be on the rise, even if the symptoms of non-celiacs sufferers are often wildly overblown (I have a wheat germ allergy, eating wheat gives me the sniffles, I eat wheat products anyways) and the gluten-free food industry helps push it.