"They did make a GTA game that sold shitloads on PS2, but now you're making an assumption that people would share the same interest in GTA as they did earlier."
What makes you think it's gone away? Just because GTA IV sold less doesn't prove GTA is less popular. Since the game has some noted changes from the PS2 games, and got some major disappointment in the user reviews, it shows that GTA IV didn't live up to what made the GTA series a smash (and Chinatown Wars obviously didn't).
The only thing that would prove GTA in the form that made it a smash can't sell on the Wii would be to put GTA in the form that made it a smash on the Wii, not recent games that had noted problems other than waning interest in the GTA brand.
Heck, the sales of both New Super Mario Bros games showed that a game series in the form that made them a smash can keep interest for years.
"I'm not saying here that a GTA game didn't sell on Wii, just that it's likely going to sell so much that it makes more sense to put on the HD systems"
"on the HD systems, where the game generates more money"
You're still making the assumption it would sell loads more which IS saying a GTA game wouldn't sell on the Wii. Since the development costs would be much lower, profit would be way higher, so selling poorly compared to the other version is the only way that it would generate more money.
"And as for the audience expansion, it would need to sell multiple millions more on Wii, than it would sell on HD systems in order to create more money than a HD game."
That is stupid. The audience expansion doesn't actually reduce the money a game makes for each copy sold. The only think that increases the money made is the higher MSRP, but that's 20%, not multiple millions. And again, the development costs will negate that.
I don't think you're doing the math at all here.
EDIT: Just to clarify, I'm calling the logic of that comment stupid, not you.