Is the idea that the Democrat's have coalesced around that number as a proposed "National Standard". In general I am not opposed to minimum wage laws with a very low floor - it seems that companies that I have worked for have always struggled to find workers in labor positions even at hourly wages slightly above the minimum, so I don't think that having a National minimum that is very low has much impact on the economy. But it would be ridiculous to assume that rural Wyoming should have the same $15/hour minimum as Seattle or California. So then you get into a quagmire where the government is constantly issuing rates indexed for cost of living and so forth.
Quite frankly, I don't care what individual cities do - if someone does not want to operate a business in Seattle, they can easily move to Tacoma, or Yakima, or Olympia - who are all free to set a wage that they feel would attract workers.