Abstract for Mr. Tabua: Three reasons for shifting the NH schedule warmer months; the success of Super League in summer; and what rugby would be competing against on TV in warmer months.
You can't have the NH rugby season over the summer, you would have no audience.
Got evidence for that?
Some counter-reasons:
One thing people bitch about in the NH is how the rugby flows at the beginning and end of the season, but slogs in those wet, heavy middle months. You're hearing more current and past players talk about how they think they could put out a better product over warmer months, and a better product would naturally attract more audience share. (Brian Moore and James Haskell had an interesting discussion about that on Full Contact a few weeks back.)
Plus the SH winter is a lot different from the NH winter -- short of a few days in the southern island of New Zealand, you don't see SH winter games with snow and sleet like you do in the north. So it's already like the SH is playing during the equivalent of the NH autumn, while the NH consistently plays in the worst conditions imaginable. Those NH conditions create more chances for poor games and injury, while the SH conditions means Super Rugby can produce more of what people say they want in the NH -- flowing, running rugby. Besides, Super Rugby starts about 4 months out from the comparably milder SH winter, while the NH domestic competitions start about a month before the weather turns to sludge. If you want to see some parity in how the game is played, the conditions the game is played in should be more similar, but the SH won't be able to re-create those same shitty NH conditions. And who really wants that?
Also, a number of players have discussed how having a unified NH/SH schedule would help spare them some wear and tear. Wasn't there a player's union appeal for that a while back? If anyone remembers that, please post a link.
So those are just some reasons why warmer-weather rugby could do well in the NH -- not evidence, just reasons, but certainly more than an assertion.
As for league's dwindling audiences in the NH, which came first, the loss of audience or the move to a summer schedule? Were they already losing audience share and moved to summer so they wouldn't have to compete against union and football? If so, then that suggests the code was already losing audience share before the move, not that the move caused the loss in audience share. Plus they're also competing against the NRL now for attention, and few people would argue that Super League is more competitive than the NRL -- so in that race the NRL is going to get more attention than Super League (and they do).
As for what rugby would be competing against in warmer months, there's almost no audience for cricket right now as it is -- certainly not in the stands, not sure about television. I don't think rugby would have much to fear from them. If you're talking test matches, the rugby could be a nice respite between sessions. And rugby's already competing against football. Its big competition would be Super League and the general rugby fan's attention would be split between NH competition and Super Rugby.
It's a known unknown, but I think there's more at stake than just a couple of NH clubs making more money.