I must admit that I don’t fully understand the “junk food is cheaper” argument.
Junk food is easier.
For the great unwashed, they either can’t or won’t cook, so microwaved rubbish it is.
For the less workshy, we’re now in a situation where both partners have to go out to work to make ends meet, so there’s no-one at home to spend two hours cooking every evening. The last thing most people want to do after being out of the house for twelve hours straight is start chopping veg.
The only way round this really is to cook in bulk and freeze your own “ready meals,” but that requires planning, budgeting, and education. The only other way you’re going to get people cooking again is to pass a “one working adult per household” law and double the wage of everyone who’s left. That or remove a zero from the value of every house in the country so that people can afford to live without running themselves into the ground. And I can’t see either of those being popular policies.