Right. D&D is the center of gravity for RPGs. There's no denying that. The question isn't "why is D&D the main intro RPG?" the question is "why do people stick with D&D and ignore basically every other RPG out there?" Especially when those other games do most things (if not everything) smoother, better, faster, and with less cruft than D&D. At some point it really is simple brand loyalty. Which, to me, is the same as conflating RPGs with D&D proper.
That's only the game play loop of WotC D&D, not how TSR D&D played at all. Besides, video games do that exact game play loop far, far better. So if that's the draw of D&D, then why are people still playing D&D? If that loop if the main draw (I don't think it is), then they should be seeking the best iteration of that loop. Hint: it's not D&D. So it can't be that.
Of course. And D&D is so dominant that it has a greater market share than all other RPGs combined. There's clearly some draw. Since the game play loop clearly isn't it. And since all RPGs have the same fundamental "your only limit is your imagination" premise, it's clearly not that either.
That's not what's being said. What's being said is people are conflating the uniqueness of RPGs, that "your only limit is your imagination" premise, with D&D itself. That's clearly not the case as all RPGs by definition have that as their starting point, so there must be something about D&D that's unique. It's not that premise. It's clearly not brilliance of game design. It's not "does combat well," it doesn't. It's not "does non-combat well," it laughably doesn't. It's not the game play loop you mentioned, video games do that infinitely better. What we have left is inertia, gravity, and popularity. Name recognition and branding. People play D&D because other people play D&D.
I think you're mistaken to dismiss the gameplay loop just because video games do it better. The crux of your response is built upon that dismissal, but I couldn't disagree more with your assumption.
Sure, there's an argument to be made for video games doing the loop better. However, it completely overlooks the fact that even the best CRPGs are lacking the strengths of TTRPGs. Namely, freedom and real intelligence behind the world. A DM can respond in ways that a computer simply can't, since computers run programs and programs can only do what a programmer programmed them to be able to do. If the programmer didn't program in a way for you to try to kick in doors, then there's no way for you to do that, irrespective of how much sense it might make to do so.
The strength of D&D is the gameplay loop within the context of that freedom and real intelligence. And, I think, that is also why players often respond very poorly to bad railroading techniques and the like. Because once you start down that railroad they really are better off playing a video game. You hear this sentiment not infrequently when the topic of railroading is raised (if the DM is going to do that, I may as well play a video game).
I think that you could argue that there are some other TTRPGs that do the loop just as well as D&D, but none (IME) that are vastly better WRT that loop. Add to that the fact that D&D has a natural inertia from being many people's first game and, if I'm right about the loop being a significant reason for players staying, it seems quite reasonable that they continue to play D&D, even if they try other games as well.
I don't disagree that people play D&D because other people play D&D. That's certainly a part of its inertia. However, I think that if that were the entirety of it, you'd see far more people abandon D&D for "better" games in the long term. Most long-time gamers I know have played other games, despite that D&D is often their game of choice.
Your food metaphor from earlier is flawed. There's a not-insignificant amount of overhead in learning a new system. It's akin to if, before dining at that fine cusine restaurant, you had to take a 101 course on fine dining at your local college (or at least a weekend seminar). Sure, you can learn while you play if someone knows the rules, but someone has to be the first to learn them so that they can teach it. Moreover, everyone is likely to have a better time if they have at least some familiarity with the rules, because then you can focus more on enjoying the game and less on learning the rules.
IMO, people stay with D&D because it is fun. You can compare D&D to McDonald's all you like, but to me it makes you seem like you're just looking down your nose at it. Personally speaking, I don't find that type of argument terribly persuasive. That said, I expect my odds of persuading you are approaching zero. I've said what I felt was salient to this topic, so I think I will bow out of the conversation, at least for the time being.