It is true that your theory appeared to me to fall in the category of damning with faint praise. We seem to agree that 5E achieves broad appeal. But there is a strong sense - in your comparing it with CF - that you believe it achieves that broad appeal by compromising the quality of design. It might be that you are instead saying something about the players, but then you would seem to be denigrating their ability to discern good design. It lands in about the same place. If that is not your intent, then I will accept that on face value.
My point is that 5E achieves its broad appeal because of the high quality of its game design, and players choose it because they are capable of appreciating that.
Mmmm. Not exactly.
This is where we get into a slightly different area. A thing can be "high quality," and have "broad appeal," and still make compromises- in fact, I'd argue that making compromises is part of the broad appeal.
I think 5e is very well designed. But because it's not a niche product, it has to make compromises. To use one example that I think everyone can agree with ...
5e includes legacy components. It has to use "parts" (rules, lore) from older editions. If the game designers were designing from scratch, if they were making some "white room" best game ever, I'm guessing some of those would be ditched. Which ones- alignment? The six ability scores? The weird mishmash of classes? The half-orc? Who knows? One person's sacred cow is another person's hamburger.
The point of this is that part of the broad appeal of the game, part of the "popularity" is that they didn't remove the legacy. There is something for everyone (or for most people).
You can use this with many aspects of the game. To be clear- I really like 5e, and I think it's an incredibly tough thing to design for broad appeal. It's easy to design something when you're only designing for a small group, and don't have to worry about sales, or popularity.