The thing that's stopping us from having a professional league isn't population, it's the fact that we have a very competitive sporting market. If you think about it a city like Dunedin with a population of 127,000 has two professional rugby teams, a professional cricket team, as well as semi-professional netball, womens rugby, football, etc.
Compare that to other markets where a city like Austin can have a population of more than two million people and yet have no (current) professional teams.