I believe the leading hypothesis is that being rich absolves you of the stressors that make junk food attractive. Others are, as you mentioned, the ability to buy more expensive food with more nourishment/satiation per calorie ("protein leverage hypothesis"), or access to nature and refuge from pollution (it's certainly the rich people where I live who ride bicycles.).
Regardless, a poor person from 50 years ago couldn't buy the cheap sugary crap even if they wanted to.
That is, the point of the parent's post was not obviously "crap food is so much cheaper than high quality food", but as most naturally read as "because we can afford more food, we're buying and eating more"
My point was "if that logic holds, I would expect the rich to be significantly fatter than the poor". Which, as you note, is not the case. Nothing more.
Spending has dropped because there are such large subsidies to cheap carbs, and that is the bulk of many people's diets. A steak and veggies is no cheaper than it was 40-50 years ago, but we're eating far more refined junk with additives because it is cheap and convenient.