My own experience is anecdotal, I realize and others may have had very different experiences. When I finally assembled what I thought would be my birding combo for years to come, i.e., the 7D2 and 100-400L ii, I couldn't have been more pleased. I used that combination for 10 months and then my 7D2 started acting up. Long story short, I bought a 1D IV. Now, I was seeing something in the files that was better than anything I'd seen from my photographs before but I do know a bit about confirmation bias (years in the audiophile crowd) so I didn't make a big deal about it around the house.
I was really debating over keeping the 1D IV or the new 7D2 Canon sent me. I couldn't afford both at the time so I struggled with it for a few weeks, asking around POTN for advice, etc. The final decision was actually made by my wife who couldn't care less about gear. She was looking at some of the photographs I'd been taking, after I'd post processed them and said, "Wow.... what did you change?" She ended up seeing what I was seeing, not knowing what camera I was using. I know lots of people here have had both the 1D IV and the 7D2. Some have kept both, many have sold their 1D IV and kept the 7D2. I realize the 1D IV is not a full frame camera but I will say this, at ISO's at or less than 3200, it's hard for me to distinguish between the photographs from my 6Dc and the 1D IV. I have had the 1D IV for 2 years and the 6Dc for only a couple months but at this point, I only reach for the 6Dc if I know the ISO will have to be 6400 or greater and that the light will be limited.
I know the 80D is getting a lot of positive press from Canon users on POTN and evidently for good reason. However, I can still see a difference between the photographs from it and the 5D IV (and even the 5D3) as well as both 6D's, at least, based on the photos posted on POTN. I might add, not every photograph, but enough to give me pause. That being said, I agree that good enough is always good enough. I'm sure all crop cameras are not created equal, just like no two full frame camera models are the same.
I can only speak for myself: I wish Canon would come out with a new APS-H 1D and that I'd win the lottery so I could afford one.
Kim (the male variety) Canon 1DX2 | 1D IV | 16-35 f/4 IS | 24-105 f/4 IS | 100L IS macro | 70-200mm f/2.8L IS II | 100-400Lii | 50 f/1.8 STM | Canon 1.4X III
RRS tripod and monopod | 580EXII | Cinch 1 & Loop 3 Special Edition | Editing Encouraged