You're basically asking one of two things: "How could the Wii have stopped the HD consoles from selling?" or, "How can the Wii go on to sell 300m?"
No matter what the Wii did, a ton of people would have bought an HD console. They have most of the big budget third party support, nearly every big franchise from the PS2 era, and its visuals look significantly better than the Wii's. The 360 has even managed a lower price point than the Wii. Even if the Wii did some amazing things, many core gamers would still probably pick up an HD console.
As for the Wii going on to sell 300m+ consoles... yeah, that's not going to happen.
Note that both PS1 and PS2's dominance had the console launching 1-2 years ahead of its closest competitor. I mean, if the Wii launched in 2004 and took all the third party support early on, I guess it'd make people much more reluctant to buy the less popular consoles. They would have to prevent the popular franchises of last gen from ending up on the HD consoles (GTA, FF, MGS, RE). Still, even that may not be enough...
