I have thought about this quite a bit and tried to word my initial post as a personal statement (not an objective fact of the inferiority of making $50k in Kansas as opposed to Quebec).
I entirely agree with your point that American families in Kansas, et al. seem reasonably content and the kind of fears that would lead me to look to Canada over the U.S. if I made $50-100k are perhaps entirely irrational.
My response though - again personal in nature, not objective in any sense - would be:
Canadians grew up in Canada with an imperfect healthcare system. Nevertheless, it's there and assessable whenever it is needed. There's no fear of random bills or imperfect coverage (there is a fear of waiting weeks or months, however, which is perhaps more important).
Canadian immigrants only understand the U.S. healthcare system in the abstract -- so losing your health insurance or having it and getting dinged with a large bill is a bit of a boogeyman (scary, but perhaps only because we don't fully understand how it all works in practice).
Likewise K-12 and college in Canada is a much simpler affair. I grew up in a city of 100,000. We had no private schools so everyone went to K-12 together. When it came time to go to university, my top-ranked school (in Canada) cost $7,000 a year and was quite easy to get into.
So for a middle-class Canadian family the notion of setting up your child to have the very best Canadian education available isn't an overly stressful affair. K-12 is public and college is reasonably affordable with reasonably high acceptance rates. Importantly: even if private K-12 is available, it's not overly difficult to get into McGill, et al. so why bother?
For a middle-class American family - because private K-12 is available and likely helps with getting into the best colleges - you feel somewhat compelled to send your child there (lest you do them a disservice). Then - again because elite colleges are available and better than privates - you feel compelled to help your child get into and pay for an elite, private education (even the top public options are quite pricey).
In reality what America provides is better alternatives. An issue I see with many colleagues with children is spending $10k a year on private K-12, but feeling like a bad parent because they can't afford the $30k option that has superior outcomes (a classic urban insecurity that is likely not entirely rational). In Canada there are few choices (or in health care only one choice) and while they are inferior to American comparables, they are much more widely assessable. You send your child to the public K-12, because that's just what's available. Your child has just one healthcare option. Your child can only pick from four "elite" universities and if she or he applies to them all, will likely get into one (at a reasonable cost).
It's all the paradox of choice, I suppose.