Don't people eat more healthy than they did 50 years ago? Weren't microwave dinners a big thing in the 70s?
Also, not all ready meals are crap. You can buy premium frozen meals from restaurant suppliers but they aren't cheap.
The main difference is that TV dinners were designed to be heated in an oven rather than a microwave, back when microwaves were less common.
One example: long ago I used to buy Bush's baked beans in a can. They had a vegetarian version which I assumed was healthier, and it even tasted better than the original. But one day I compared the labels and found the vegetarian version had more added sugar and more calories per serving.
We were fed a massive amount of misinformation about healthy foods in the 1980s. Hopefully things will improve from now on.