I think we're wrong to assume all football players want to be live football spectators.
Actually, just as many hardcore Phoenix fans don't or have never played football or watched football on TV
I think partly there's needs to be more focus on how great going to live sport is. That's partly about winning, partly about feeling connected to the team and partly about the match day experience - all of these can be improved on
You're right, but I still think the gap between 18,000 registered players in Wellington (plus their families) and a 7-8,000 average attendance for the Nix is worth properly investigating. In fact, your observation that not all of the 8,000 currently going to watch the Nix also play football just makes the size of the gap even greater, so even more worth investigating.
If half of the Nix current home average crowd also play football (and even that may be an over-estimate) then that's only 22% of Capital Football's registered players. Shift that to 30% and you get an extra 1,500 spectators.
That said, winning regularly at home, which also greatly increases your chances of making the playoffs where the big money home games are, is still by far the most significant factor in the equation.