All they need to do is go back to their old recipes and ingredients from 1960s and 1970s even the first half of 1980s, and they would be back. There is no mystery to their downfall — their food simply became inedible junk. Their famous fries have gone down the worst with their crappy fake tallow and cardboard-like pesticides laden GMO potatoes.
Remember when McDonald's hired local high school kids and one out of five families had a teen who had worked there at one point? Always a friendly face. Now they've gone with 'professional washed up gang bangers' and it's not a nice experience. Nothing about McDonalds is good anymore - not the food, not the service, not the atmosphere, not the old community involvement. It's run by white liberal elites and it's unpleasant.
The food didn’t become inedible, it really hasn’t changed that much from what it was in the old days. But we’ve become smarter in the ways of food and realized their food was ALWAYS inedible. McD’s success was in the days when we knew instant coffee tasted like crap and we drank it anyway. We don’t do that anymore.