None of your iPads were perfect; they just fell within your standards.

I'm only half kidding. This is a mass produced consumer electronic device, not a finely hand-crafted masterpiece. They are all built within a range of acceptable tolerances. This is clearly the case as screens from different manufacturers or lots are visibly different. These are not defects. Some will find it unacceptable but that's the product they are purchasing.
I very much disagree that QC has gone downhill lately. Apple is simply shipping more products. I've been following them for years and they've had some pretty spectacular defects over that time. I don't know the hard stats but recent products seem more durable and better performing than ever.
People should certainly return a product that was an unexpected disappointment but this ridiculous cherry picking is a bit over the top. If you know a particular product is heavy on defects (in your view) you should simply stay away and go for something that better suits your needs. Why reward a company for shipping crap?