That's somewhat similar yes, but the question isn't "is that the same" - it's "okay it might be the same, and what's the problem with that exactly?".
Why does that matter? Because by framing it like this you're already implicitly suggesting there is something wrong with either Mechanar being the most popular dungeon because of its rewards OR with DF being popular because it's convenient.
That's another flawed statement because you're implicitly suggesting that there's some kind of clear (if not strict) separation between fun and performance.
Which just isn't how it works.
To many people, it's fun BECAUSE it performs so well. And your question somehow categorically undermines that by suggesting there's either fun or performance, and that somehow these are separately evaluated.
THAT'S EXTREMELY FALLACIOUS REASONING.
Now you're making an entirely different argument.
For this, you'd have to actually DEMONSTRATE that something like DF is ACTUALLY a worse experience.
You're just assuming this wildly with no evidence or demonstration; in fact, you're actively positing this IN THE FACE OF POPULARITY, so you can't even argue that more people like it (because they clearly don't), and that makes it tricky to find a good metric for what is and is not "fun".
I'm not using "good" anywhere. You are. All I'm saying is that if in a hypothetical scenario participation numbers turn out a certain way, then that's clearly where player preference lies. If your argument is "X is more fun" but if given the choice between X and Y the vast majority don't actually choose X, then how did you determine that X is, in fact, more fun? That's precisely what I've tried to expose in my example. And it also doesn't mean (and I'm not saying it anywhere) that in such a hypothetical it follows that Y must be "more fun" - only that it's more popular.
The problem really boils down to some people thinking they understand what "fun" means and how it's measured, and that because of that, they get to determine what should be fun or more fun, even if it goes against the popular choice. I'm asking 1. where that confidence is coming from; and 2. what gives them the right to make those determinations for other people. The fact that very often what they think is most fun also just so happens to coincide with their personal preference makes me very suspicious.
I get that developers have the final say and we can't really question them on authority; only on result. But for people to defend things as though their personal preference was somehow sanctified as "officially" fun/more fun because it coincides with dev decisions is nothing but pure hubris.