"why should I spend 10 million to make a port Assassin's Creed when I can spend less than half, make a party game and sell more"
What proof will it sell less? You kind of need AC games on the Wii to prove they will sell less. Not other games you lump in there.
I think the real test on what exactly sells well for Ubisoft will be Prince of Persia: The Forgotten Sands. Avatar pretty much bombed on all consoles so I think PoP will be a good game to see how well it ends up.