Sorry the Northen Hemiphere argument is nonsence when it comes to Japan
6 Nations travel distance
Ireland to Japan - 5934.856
England to Japan - 5854.816
France to Japan - 6127.427
Italy to Japan - 6051.904
Scotland to Japan - 5671.516
Wales to Japan - 5905.438
Rugby championship travel distance
New Zealand to Japan - 5817.679 (less than any 6 nations team)
Australia to Japan - 4262.601 (significantly less than any 6 nations team)
Argentina to Japan - 11240.762 (far further than any 6 nations team)
South Africa to Japan - 8762.960 (significantly further than any 6 nations team)
However to put this into some kind context South Africa would travel 5053.600 to Italy, 4780.810 to Argentina, 6228.860 to Scotland, 7192.057 to New Zealand. So it's further than any match they would normally travel for. Argentina is a ridiculous distance away for any match they would normally play as well.
Scotland however would travel 11371.573 to New Zealand.
Nobody thinks of the Japan tournamenet as a NH tournamenet as it is not one of the 6 Nations teams. It will not be 3 NH cups in a row to anyone really in reality it's more of third group which is Japan and the Pacific Nations. None of the voting committee representing the 6 Nations will consider it has three in a row.
The might consider the fact that it will be a minimum of 16 years since a RC nation has got the tournament though if Ireland do get it as opposed to 8 for the 6N. But I doubt it they are more likely to think of Japan as SH tournament.