Nintendo can only do so much on their own. Without the help of third parties that will be impossible, because they will have to go through long droughts and momentum is then lost and people just moves on to the consoles where the games are, like it happened after 2008.
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..