I would not say that EA's MySims is a simple game. In fact it looks to be a complex game by most metrics. It allows for user generated content, it has tons of customizability, it has complex computer-human relationships, etc. MySims is also aimed for a more casual market than the usual "hardcore" staple. I think MySims demonstrates that a complex game can be made with to work on the Wii. In fact I think it goes further and demonstrates how a complex game built for older input schemes can be adapted to work on the Wii.
As for your second claim about big comanies focusing on casual games. I think one look at the DS will reveal that that fear is unfounded. In fact, looking at the US market, you did say US companies after all, I see very few "casual" games on the DS. Comparing the US and Japanese markets side-by-side I see very few in one market and a growing mountain of casual games in the other. If anything American companies have failed to get on the casual games bandwagon.
I think the greatest challenge to "complex" games on the Wii will be its input mechanism. However isn't that what we all already knew? I don't see why it should be any harder than moving from 2-d to 3-d which required throwing out many good ideas and saw the death of equally many popular franchises and game mechanics. So I would either revise your theory and say the Wii may not have the same types of complex games, or just give up on it altogether.Â