If we raise the minimum wage to $20/hr, fast food will no longer be cheap. Some McDonalds will turn into nice restaurants, but as the minimum price of going out rises, more people will seek alternatives such as cooking at home (likely paying themselves a lot less than $20/hr, ironically). The net effect is to destroy an industry.
You can pity someone working for $4/hr doing hard agricultural work. But if you apply minimum wage to the situation, you don't raise his wage, you destroy his job. He likely needs the money; that doesn't help!
Hence, I have come to regard minimum wage as an act of oppression, and unmitigatedly evil. It should be abolished.