McDonalds doesn't pay $15-20/h.
If you live somewhere nice enough that McDonalds and Grocery Stores can't maintain a functional staff, then what they are willing to pay is not in line with what it costs to live and work in your area. Actually, UBI might help them hire more easily.
However, in 20 years each McDonalds store will only have one or two employees. All of the jobs that you're talking about are GOING AWAY. The people who have those jobs now are completely fucked. UBI isn't about right now, it's about trends that are happening right now and are expected to massively change the employment landscape.