Poor and sub-par IQ can both be defined and quantified in a reasonable manner.
2. worse than is usual, expected, or desirable; of a low or inferior standard or quality
What is 'usual'? Expected or desirable by whom? Still a value judgement. Who sets the standard for image quality?
You're correct that people can come here to vent, but they cannot expect everyone to agree with their position. I suspect you'll find a few people here who disagree with your belief that Canon sensors produce images of a low or inferior quality. Maybe more than a few?
Rent a D810, shoot and post all the RAW images you want, I bet you find that the Exmor images have more DR and less noise in the shadows, especially when those shadows are pushed. But we know that already. Will you prove it actually matters to others? That's the real issue. We already know it doesn't seem to matter to Canon, as far as can be judged by their sensors over the past several years.
There's more to image quality than the pixels that make up the sensor. That's an argument that I frequently use to refute those who claim that Canon must improve their sensors. I have to say, you summed up that argument very eloquently...
tell me Jrista, image quality, what is it more than the sensor and the measurements for example DXO does?
color quality/ color resolution
noise and high iso properties
Proper focus (AF system)
Non-blurry frame (FPS/AF system)
Correct frame (FPS)
Pixels on subject (pixel density)
Subject size, clarity, and noise levels in cropped frame (pixel density)
If any of the above are wrong, it doesn't really matter how good the sensor is. There is more to IQ than just the technical design of each pixel in a sensor.
I've never stopped believing there are other factors that affect IQ. The thing that's changed, is I realize that there are limits to how much those things can improve IQ. One you get a clearly focused shot (AF), once you get the right shot of the right moment (FPS), once you gather enough detail (pixel density)...then the only thing left that can improve your IQ is the quality of the data. Even at high ISO now, it seems that things can be improved...quite significantly even (something I honestly did not think was possible...not to the degree it's been done.)
There are also certain kinds of photography where the AF system, frame rate, even the meter, are quite meaningless. So regardless of whether you have the best or the worst AF system or meter...eh, it simply doesn't matter. Image quality still matters, though...and when all the other factors become meaningless, that one becomes particularly meaningful.
While I think Canon may not long hold onto the title, a title I do believe they still hold, for best AF system or fastest frame rates, they currently do excel at those things. Their metering was a bit lacking in some ways, however as they appear to be trickling down iTR (at least to pro-grade cameras), I think that problem has been solved. They have decently high resolution sensors, certainly nothing to complain about on that front, and it's likely there will be increases in pixel density with the 5D IV and subsequent cameras.
Canon already does extremely well our flat out excels at all the other factors that have a significant impact on IQ. I don't have any issues with the 5D III AF, it's frame rate is good (could be better, but no slouch), it's pixel density is good, and when framed well, it's high ISO IQ is great. So, yeah...I think Canon nailed all the other aspects I listed when replying to Mikael there. There isn't anything to complain about on that front.
Canon delivered, and when it comes to action shooting...I still don't think there is a better place to go (yet...I'm still pretty impressed with the new AF system from Sony, and the A7s has demonstrated that there are still some very amazing things that can be done on the high ISO front...adding two more stops of DR at ISO 51200 is pretty surprising to me).
There is only one area where Canon doesn't do well...one area where they fall farther behind the competition every time the competition releases a new camera. The term-that-shall-not-be-spoken is the the one area where Canon has some significant room to improve. It's the only thing left to complain about...and I really, really, really, REALLY, for a very, very, very, VERY long time...have wanted more TTSNBS. In a Canon
I think it's sad that people like me are forced to look elsewhere to satisfy their needs. Seems that's the case though. Just don't expect us to go quietly...we'll kick and scratch and scream until Canon finally delivers on the TTSNBS front, because we love Canon.