Your advice is a generation or two off, unfortunately.

Most people's grandparents were running households in the middle of the 20th century or later, and were already deep in the transition towards processed foods. Box cake, TV dinner, casseroles made from canned soup, canned/bottled soda, margarine... it all starts with the boom of consumerism, advertising, and expanded workforce in the second quarter of the 20th century and essentially becomes completely normalized by the fourth.

Your grandparents may still have known how to cook and eat the old way, and might have done it sometimes. but they were told they didn't need to bother and that the days of needing to were behind them, so they didn't do it as much and they didn't make sure their kids or grandkids even knew how.

Hell no my grandparents ate like crap compared to what's available today in the supermarket! Modern agriculture and transportation gives me far better options than they had.

Your advice will probably work for some age groups, but it doesn't for mine, unfortunately.

A whole lot of peoples grandparents had horrendous diets (and died young!)

Absolutely not, my grandparents all had diabetes, heart disease, and just generally poor health for many decades. They all ate like complete shit and we're super stubborn about it - wearing it like a badge of honor.

I eat much healthier than my grandparents.