I think we make the calculation from the wrong direction.
Can be the wii graphics more advanced that the gc?
Yes.
had they the resorces and the time for it?
Yes.
Why they don't use it?
I think the answer is in the difierent design philosophy.
In the case of the ps3 the sony is started the system planning from the cpu+bd+gpu, in the case of the xb2 the system desing is started from the live+gpu+cpu, in the wii it is started from the controller+low cost+small form factor.
Due to this,in the case of ps3/xb2 the design is started with the definition of the graphic system, and the first tool that was received by the soft devs was the graphics tool.
In the case of the wii the first tool was the controller,and the last(probably just before the console relase,because it grant the best price/performance ratio) was the grahics subsystem.
The backward compatibility was important,possibly because when the release started they had only the gc tools.
And for me it mean that the graphic part of the wii will evolve dramaticly in the future,compared to the evolution of the ps3 or xb2.