We already have a traffic light system on foods and it is largely ignored, partly because few people have the time and energy but also because you have to look at your diet as a whole and not judge it by individual items. I also think these UPF studies have a bit of a conservative agenda.
Why do people eat more processed food? Because there isn't a homemaker spending hours a day preparing meals as a full time job. Proceesed and convenience foods are massively egalitarian and I think let more people join the workforce.
I mean different countries different people making the decision.