only consider one side. But that being said (and I lean right), I don't think we've seen the full impact of the law in Seattle yet. The full minimum wage increase has not yet gone fully into effect yet. Smaller businesses won't reach it for a few years yet while some will hit the full $15 next year. And even when the full pay increase goes into effect, I think it will take some time to understand the overall impact (whether net positive, negative, or neutral). So anyone saying that they know anything one way or another is making it up.
But, history has shown that government's involvement in markets is risky and causes inequities.