Corporations, like McDonald’s, are in the business to make money, not lose it. If they made 10 billion dollars last year they aren’t going to make 9 billion and be satisfied.
If they are forced to pay 15.00 dollars an hour they will either, lay off a bunch of workers and replace them with kiosks or they will raise the price of their products to make up the difference, which means they will have less customers to buy their 6.00 burgers.
They have already installed some kiosks in some of their restaurants in anticipation of the pay hike.
Which ever way this goes, the workers lose.