McDonalds is a good analogy, I think, but what I'm trying to express does not have much to do with quality... it is more like ubiquity + ignorance. How people can be experts in 5e without ever hearing about DCC (or CoC or GURPS for that matter). And, look, I cannot say any of those games are 100% better than 5e. I just think it is incredibly USEFUL to learn about RPGs, not only 5e D&D.
Heck, even reading Moldvay's Basic - a 50ish-page book - sounds infinitely more useful than getting a 200+ page book with new ideas on how to use your bonus action. If you want something more current, Shadow of the Demon Lord, DCC RPG...
The campaigns bother me the most, because I tried running them, and it is a hassle. While the character options are somewhat balanced and play-test (but yes, there is some power creep and caster supremacy gets worse and worse), there are essential parts of the campaigns that have obvious mistakes (railroading, misunderstanding hexcrawls, etc.). But, I mean, people are still running them and enjoying, so...
I hear you and feel your pain - but again, you and I are not the drones WotC is looking for. They're looking at their new Millenial player base, and upcoming Gen Zers who know nothing of Moldvay or THACO or the OSR. They might eventually.
In that sense, the McDonalds analogy might work insofar as both McDs and D&D are the "gateway drugs" to fast food and RPGs, respectively.
Many people stop at McDonalds because it is cheap and, frankly, tasty in a junk food way. Some try other fast food joints and restaurants, perhaps gradually fine-tuning their tastes. A smaller percentage get into cooking and start making their own burgers.
But the majority of people stop somewhere before that, and settle on their "brand" - be it McDonalds, Five Guys, or D&D. It is a small percentage that get into boutique games or even creating their own.
So while I agree that people are only enriched by going deeper than D&D (or 5E), I also understand why they--especially casual players--just stick with the official material.
EDIT: here is another analogy: a body-builder with strong arms who refuses to train his legs... ever. Nice arms, but training your legs would be really helpful overall.
I don't think that quite works, because it implies that 5E-only folks are "doing it wrong." Or rather, if we want to use the analogy of body-building, I think the equivalent is someone who just goes to the gym 2-3 times a week for 45 minutes to maintain a decent physique, but doesn't get into it in any great depth.