Staffing is more complicated than "how much they sold during their shift"?
Yes, staffing is more complicated, but min-wage labor cost for a restaurant are a significant portion of the costs and profit margins are very thin [0]. Many won't be able to absorb a ~50% increase in labor cost.
[0] http://www.sfgate.com/bayarea/article/Economics-of-running-a...
Its easy to play 'dot-to-dot' with these things - higher cost means lower sales means people fired. OR more money in the working class pocket means more coffee purchases means more hiring. You can connect-the-dots lots of ways.
The truth is, some of everything happens, and then it settles down into something workable. Yes we'll pay more for our coffee, but we'll also have more money in our pockets. And so on.