On the other hand, to those of us not 24/7 paragons, the goddamn store being deliberately set up to stimulate three year olds to beg for sugar is a pain in the ass.
Your examples are way harder. Never raise your voice in anger? That's unachievable and absurd for most people. Take ten minutes per week to work with a child to improve unacceptable behavior? It sucks but is pretty realistic.
Sometimes you have no choice but to drag an already tired kid into the store, in a hurry, with seven things on your mind and after a long day, and the teachable moment may be out of reach. And then the kid-eye-level candy bars are very annoying.
You write "It's pretty obvious to any parent but very hard to do 100% of the time" but I don't think my method requires that. It could be more like - "I have some free time today, I'll take my kid to the store to help teach him self-control". That is, you can still be upset with store designers that they arranged their items to cause your kids to frustrate you, and you can still usually leave your kid at home while you go to the store, but you can also sometimes take advantage of the environment that the store has created for you.
Similarly, I was listening to a podcast recently about the moral virtue of becoming a vegetarian due to the various evils caused by factory farming. It's one thing that I've always struggled with, because while I see the moral case, I really like the taste of meat. One point that was made on this podcast was: Eating 10% less meat is about 10% as good as being a vegetarian. And, if you can't force yourself to become a vegetarian, it is better to be 10% of a vegetarian than 0%. Likewise, applying rigorous parenting techniques may be too challenging to do consistently, but applying them 10% of the time may be 10% as good - but better than nothing!
Of course, reality is more like your most recent paragraph, where we do what we can and every little counts. We don't disagree.
After a few times of that, the kid will come to the conclusion that (s)he won't get it and just forget it. It's the same way you train your body in response to certain desires.
In the supermarket I need to bit my tongue when I see a parent doing it all wrong. They are terrified when their children start to throw a tantrum. And of course the children smell the fear and do it every time.
I offer a complicity nod when I see a smarter mother acting as if it's nothing and smiling even when the child is rolling on the floor crying.