A few things though.. For one, it's not like Wii's 3rd party support is non exsitent in Japan. There are some decent titles like 2 Dragon Quest games, Last Story, Xenoblade, Line Attack Heroes, etc., and Red Steel 2 was just released, though with poor sales..
But also, it's not like PS3 has an incredible amount of 3rd party games being released their either. The amount of games coming out, last I checked, were barely more than Wii's. And of course, Xbox 360 is pretty much irrelivant in Japan.
It's not an issue of the Wii specifically, and its 3rd party support. It's more a matter of console gaming in general there, which has taken a backseat to portables and continues to decline.
Of course more 3rd party support couldn't hurt, but I honestly don't think Nintendo needs them to keep the Wii going in Japan. They got through it fine during the N64 days, and add this to the fact that Japan is a shrinking market, Nintendo could probably string along the Wii over there solely on 1st and 2nd party titles if they wanted. 3rd parties would just release more shovelware anyway, so best it's left off the console.
Nintendo also has plenty of developement studios to keep the games coming, it's not like they have a 10 person team to pump out all their games..
Xenoblade, The Last Story and Line Attack Heroes are 1st party. Line Attack Heros doesn't even have a date there and I really doubt it will sell more than 50k judging by the sales of other smaller 1st party games.
PS3 does really have more games than the Wii, like 3 times more upcoming third party games than the Wii, I can PM you or post in your wall the list which BTW I update on a daily basis.
Like I said before, home consoles declined because of bad decisions made by the devs, not because users don't care for them anymore. Wii was a huge success during it's first 2 years, as was PS2 during all its life, but Wii's momentum was lost because devs ignored it for the most part and some are even labeling it as a dead console.
If Nintendo didn't need 3rd party support Wii sales would have never declined, but we know that Nintendo aren't perfect and 2 badly planned releases killed the whole momentum (AC:CF and Wii Music) there was no recovery after that. Had the Wii had strong 3rd party support they would have managed to get through relatively well, but reality is it was a complete disaster.
Nintendo has many tams, true, but they also have 2 (soon 3) consoles to develop for, and they even admitted they managed their resources badly and delayed a lot of games. Again, Nintendo isn't perfect, so one mistake they make and it all goes bad, simply because they are the only ones keeping the Wii alive.
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tag - "I wouldn't trust gamespot, even if it was a live comparison."
Bets with Conegamer:
Pandora's Tower will have an opening week of less than 37k in Japan. (Won!)
Pandora's Tower will sell less than 100k lifetime in Japan.
Stakes: 1 week of avatar control for each one.