In addition to celiac disease, which is not actually so rare - only about 3 times as rare as red hair - there is also Wheat Allergy, which is very real and detectable by measuring immune responses - and Non-Celiac Gluten Sensitivity (NCGS).
Most of the "health fad" people you're talking about would probably self-categorize themselves as NCGS, but that doesn't mean that everybody who is NCGS is part of a health fad. Double-blind "challenge" studies where people are unaware if what they are eating contains wheat/gluten have confirmed that for some people this is definitely real and not a made up fad.
I'm not denying the existence of people going gluten free for fad or orthorexia reasons, but be careful when you lump everyone who is gluten free for non-celiac reasons into that category.
I work with a guy who is a real, fully diagnosed celiac sufferer. The gluten-free fad has had serious consequences for him. Some restaurants don't take him seriously, claim gluten-free food when it isn't, and he gets seriously ill when that happens. He doesn't eat out so much any more because it's not worth the risk.
It absolutely increased availability. When my sibling was first diagnosed things like gluten free bread had to be ordered and shipped and were extremely expensive. Now they're available in many (mostly up-market) grocery stores, and while still expensive have come down in price a lot.
I’m currently on vacation and the hotel breakfast buffet has a gluten free bread option. Granted, it’s prepackaged toast, but I’d say it’s better than hungry.
A close friend of mine with celiac disease says it was a whole different world after 2014 or so. Like being on an alien planet and being in a familiar place.
There are plenty of foods that have never had gluten-containing ingredients, but have always been made in factories or contexts where they come into contact with gluten. Celiacs, and people with NCGS who have strong reactions, when given the option will usually avoid anything that hasn't been actively tested and confirmed gluten free. For things like a bag of rice, or produce, there isn't an option with that label for re-assurance, and then it's a gamble... the odds of a bag of rice containing gluten may be low, but with such big consequences it's still a gamble. So the proliferation of the GF label is useful. There have also been a ton of new products launched that are gluten free, so it's definitely not just a labeling thing.
