If all you want is PS2 games, Wii is definitely an easy port machine. I somehow doubt the market will be pleased with this, though.
Porting to Gamecube was cheaper than porting to Wii will be, and that's even assuming a Wii port is possible, which it won't always be.
Nobody's taking Nintendo's online plans seriously. I'm having trouble even taking Sony seriously.
Gamecube had pretty similar third party support to the Wii. They were always third in line and will still be third in line, but there's enough support there to supplement those primarily interested in Nintendo's games.
I'd suggest that Wii Play and Sports are the most convincing argument that quality isn't really as great an influence on sales as sometimes is suggested.