I know they do. But once you already have an OCed 1080, you're already reaching what, 120-130 FPS on a GTX 1080 at 1440p in BF1? And mind you, that's one of the most demanding first person shooters out there, the kind of games where you do really want to achieve high FPS on. And that's assuming they have all settings cranked on ultra.
So it's not like you are missing out on low frame rates in 1440p with a GTX 1080.
You are saying that's it's reasonable to assume that the overwhelming majority of GTX 1080 users are on 1080p displays? That's a bold statement. The people I know with that card all play at 1440p. Even with GTX 1070, most use 1440p (though a GTX 1070 was never part of this discussion). It actually makes zero sense to purchase a 1080 for 1080p to begin with. Yes, I got your statement of people buying overprowered stuff. And though I agree about this when it comes to CPU (mainly because of how misrepresented the meaning of them are, as proven by users in this discussions and in general reviewers). But people are not baboons either. It doesn't take a rocket scientist to read benchmarks online. Nor do these people clik "buy" on such expensive cards without knowing what they're buying. Reviews for GPUs etc. are not niche. They're popular across the whole gaming segment. At least enough for someone to not cash out $500 more than he has to for what his needs are.
Your arguments are also contradicting each other. In one area you say these people are willing to pay way more than what they need for their stuff. On the other hand, they'll continue to use a 1080p display "until it breaks" (when do monitors ever "break", really?)
But of course, if you truly believe the overwhelming majority of GTX 1080 users are using 1080p displays, I'd be happy to put up a poll on this forum to see how many GTX 1080 users have 1080 displays and how many have 1440p (or 4K) displays.