They won’t have to. Their wage is tied to the minimum wage. The minimum wage goes up, their wage goes up. Now you know why Democrats want to raise it.
I have been in Union negoations for a while.
I have yet to see a contract based on a multiplier of minimum wage. It may exist, but in the industries I have played in that is not how it is. This includes states where it is already $15
Now, if the MW goes to 15, you will pressure on the next contract to go up.