A rise from $11.35 now to $15 per hour by 2021 is a rise of about 7% per year, compounded for four years for a total rise of 32%. 
Question:  Lets assume, for laughs,  that labour accounts for the entire cost of a cup of coffee.  If it rises 32% from $3.00 per cup now to $3.96 by 2021, will you stop buying it?