McDonalds is losing money on many restaurants because wages are too high, and margins are negative. While a law firm might turn a 800% profit billing out the work of paralegals, a fast food restaurant is lucky if they can make 15% net profit on a workers wage. It's very thin. Most businesses need about 100% markup from what net revenue the employee brings in, versus their salary.
There is no arguing that the majority of fast food restaurants in the US would close if the minimum wage doubles. Now, I wouldn't say that would necessarily be bad on its own, Americans would start to cook more of their meals and rely less on low quality food. The problem is that even though Americans would cook more meals, logistically, many, many, grocery stores would have to close too. Everything would be hub and spoke and delivered by Amazon, whom already are paying over $15 an hour for most of their employees. The cost of retail fronts is astronomical.
The biggest winners would be Class A drivers, whom already make far more than $15 an hour, and supply chain workers. Trucking and logistics would see a surge in business to online retailers.
You would also see a lot of prime real estate zoned for business near residential areas crater in price. The wealthy people with cash would either mop up the losses from failed business owners now in the lower class, or convert existing commercial property into residential rentals, further increasing the income disparity levels. This is exactly what has happened since the 2007-8 financial crisis. Home ownership is lower, and rich people have more property.
The working class would really lose out. The most ironic thing is that with all the low skilled jobs evaporating, there would be a ton of competition for the remaining skilled work, and wages in the $20-25 an hour range would plummet to a flat $15 everywhere. Then on top of that, with unemployment nearing 25%, you are going to have all these low skilled workers now trying to be entrepreneurs. Since they have no skills they will almost all fail, but the ones who are able to turn a profit, will now make far less than minimum wage on average. There's no governmental guarantee to small business owners that they too make $15 an hour.
The solution to income inequality is not artificially manipulating the markets like a dictatorship, but by bringing back American manufacturing jobs from China. Trump is the only one to suggest a 45% tariff. This will actually increase the average hourly greatly.