OTOH, the nice dining experiences will be done by people who can improvise and improve day-to-day based on their own tastes. eventually, the stuff people do will be the "good" stuff, and the stuff robots do will be the "bad" stuff. Hopefully. A more negative viewpoint says that robots will do the "high value" stuff and the people will only do the "low value" bad stuff.
In fact, if you consider the partial oxygen pressure (which is what matters for your lungs), this air is no worse than the air you find inside an airplane, or at a 2500m (8200 ft) mountain peak; Wikipedia says 140 million people live permanently at above 2500 m.
Now, this already exists and is called a hypoxic air fire prevention system, and is used e.g. in some data centers and in archives of valuable documents. It's not used in fast food restaurants though, because they have better ways of preventing and extinguishing fires at lower cost.
The majority of space in a McDonalds is the dining area and ordering counter area These areas are occupied by customers. The kitchens in a McDonalds is already pretty compact and dense.
I think this also has some interesting implications for real estate long term - if every mcdonalds needs 50% of the space they currently occupy over time that real estate will be put back in the market, multiply that by every organization doing that and it has some follow in implications that I can't forsee but find very interesting.