"...but I think you're confused about what I wrote."
I can't argue that point! 
But my point was just the comparison of the 90D vs the 6D Mk II. And, that somebody would bring up the low light benefit of the FF over the 90D. I wanted to make sure the OP considers the total package. The age difference and the tech.
However,
"Instead, the 2 stops I wrote about was very specific to my example of moving from a Rebel T4i to the first-gen 6D."
You may not have seen a 2-stop benefit if you had selected different lenses for instance. It is unlikely the way companies manufacture gear, you will ever see two cameras, a cropper and FF come out in nearly the same time frame with nearly the same innards. Except one has a big FF sensor with big pixels and the other a crop sensor with tiny pixels.
To give you your due, I agree FF will "usually" do better in low light all else being equal. But you always need to consider the total package. More clear I hope!
...such a user is more likely to soon get annoyed by his camera's missing features than someone who buys, say, a T7i. Much of the talk against the 6D isn't that it's an inferior camera, but just that do you really want to buy a camera that you may get tired of before you wear it out.
That's an interesting perspective. I take it you have a 6D or 6D II. As I noted, I have a 5D Mark IV, as well. It's heavy and has a number of features that I don't often use. I use my 6D II (or EOS RP) when I need to carry a lighter camera about for field nature photography and I get results that are equivalent to those of the 5D IV. I suppose that, for some, the 6D series has "missing features," but that simply never occurred to me. I've been using the 6D II since the day it went on sale and it has also never occurred to me that I would tire of it before it wears out. With four interchangeable-lens Canon bodies, I don't have to have a hard favorite. But I very often favor the 6D Mark II for its ease of use, ruggedness in the field, and high-quality raw photos. The price of lenses is not an issue for me. When looking at body specs, I pay attention to my specific needs. I do pro bono nature macrophotography for scientific and educational institutions. GPS is a must for a nature photographer. Two card slots? Not important to me in the least; I use only one of the two on my 5D IV. Video? 1080P @ 59.94 fps produces the high-quality short videos of arthropods that I sometimes make. ISO 6 zillion? Nope. 100 megapixels? Waste of money for me.
That argument can be, and sometimes is, tinged with unnecessary pomposity.
Bingo. I do photography, not pomposity. I show photos, not cameras. I'm not in the least abashed if I'm on a field trip where everyone else has a 1DX and I have brought my little M5. No one has ever looked at a photo from my M5 and said "too bad you didn't use a (whatever)." Frankly, there's not a commonly available camera that I know of, at least up through Phase One (heavy, not suitable for macrophotography), that I couldn't own if I wanted it. Ergonomics, weight, high-quality raws, ability to remotely control my Profoto B1s (which I also take into the field).
This is what I'm looking for, and the 6D Mark II doesn't disappoint. Cicada-Killer Wasp, Sphecius speciosus.
