That doesn't really explain why same-day PS2/Xbox/GC multiplatform releases, with rare exceptions, consistently sold disproportionately worst on GC. In fact, there were a number of third-party franchises like Burnout that got GC ports earlier on, but ended up becoming PS2/Xbox only due to the generally abysmal sales of multiplatform titles on the system.
Having technically competitive hardware makes multiplatform titles possible, which they mostly weren't on GC. But it won't be nearly enough to make Wii U commercially viable for those titles, I suspect.