I think Galian would have a far more convincing argument if he instead argued that platform consolidation in general is in the interest of third parties, rather than consolidation around the Playstation specifically.
He points out that some games like Monster Hunter might have been hurt by being on the Wii. I agree. However, this isn't because the Wii can't sell software -- clearly it can -- but because MH is one of only a few select titles that made the leap from Playstation exclusive last generation to Wii exclusive now.
Just as the Playstation 3 would have done better if it had MH3 announced for it, so too would the Wii have been a better software environment overall if it had a Final Fantasy, Metal Gear Solid, Devil May Cry, etc. announced for it. The PS3 had all those games announced for it at or near launch.
The enormous third party success on the DS suggests to me that third parties can succeed virtually anywhere, as long as they move en masse and don't fracture their market. Splitting up the fanbases of Monster Hunter, Final Fantasy, Dragon Quest, et al is not a good idea, because many of those customers are actually the same people. You can add other franchises like Tales and Metal Gear to this list, too.
I can imagine third parties picking a Playstation system, because they have had success there in the past, as Galian suggests. However, I can also imagine them picking a Nintendo system, because any system without Nintendo on it creates a fractured market in Japan. Any portable system that doesn't have Pokemon, Mario Kart, and NSMB is not a consolidated one.
I'm not suggesting which direction I think is the right answer, just that whichever direction they choose, it is in third parties' interest to choose it together. The strength of the software environment in the PS2 era was not because Sony systems have some magical aura that makes third party games sell better; it was because every single major third party was making their games exclusively for that system. They helped each other by all going to the same place.