I think the "parents not taking their kids to McD's" angle is big. For whatever reason, when i take my kids to McDonald's, I feel like a failure as a father. Like if my friends see me doing it they'll think "man what a loser, takes his kids to Mcdonald's". It's all in the image, I know...but that's the image they have now. Crap food that is horrible for you.
Even though I'd argue it's ten times worse to take your family to Chili's/Friday's/Outback or any of the sitdown chain restaurants that are absolutely LOADED in calories.
I think its this and there is nothing they can do about it. Once societal ostracizing sets in there's nothing you can do. Same with smoking: it took a really long time but now, unless you really don't give a crap what anyone thinks which is rare, most people are a bit ashamed to light up. I would be ashamed to feed my kids McDonalds or any fast food on a regular basis. Simply because i know, no matter what, that the food is not harvested in a healthy, sustainable way, that it's not humane, and it's not healthy for my kids. It's a double-shame burger with mega-shame fries.
There's nothing they can do to rebrand or make themselves not shameful. If they completely reformed everything: ingredients, processes, quality, and overall health quality, they would be bankrupt before the measures turned their first profit margin.
They could massively downsize and appeal to the smaller segment that really doesn't care about what they put into their body or what others think, but that's probably less than 20% of the population, really likely less than 5%.
If they bill themselves as only an occasional non-healthy treat, they can't sustain their current infrastructure.