1) Demand for fast food is probably as elastic as demand is for most things. At higher prices, demand will undoubtedly fall off -- squeezing margins and income.
2) You'd expect fast food restaurants, as Ace pointed out, to continue to push the trend of automating, and to the extent they can, do more with less in that environment. The ones that can't do that and be able to maintain any sort of margins, as **** pointed out, won't be able to stay in business., and the ones that can jump through those hoops due to their scale, obviously will be employing fewer people.
3) Which is why, the truth is that the actual minimum wage is (and will remain for even more people) $0.