Do you think even a Wii delivered by a new party would have found traction, though? Nintendo is the only player that's been around since the beginning (effectively), used to OWN the market, still had plenty of mindshare, knew to deliver their own good games first... and still the concept was considered very odd by developers, who were worried about showing large support right off the bat even through they were already well-practiced with Gamecube coding, simply because Nintendo ONLY sold as many consoles as Microsoft in the previous generation.
Do you honestly think a new player entering the market or a return by Sega (who floundered in direction, eventually drowned, and haven't been releasing top-selling games since then anyway) would be able to convince publishers OR customers who are already being stretched in three other directions? Do you think they'd be able to deliver something with Wii's style, and have Nintendo's image to carry it to term?
Even if Sega or Apple (who can certainly catch a consumer's eye) could deliver something uniquely interesting or strong, I rather doubt it could catch fire the same way, and "selling a bit" wouldn't capture enough 3rd party attention to build momentum for a platform.
It'd be a lot more feasable (and, likely, profitable) to build a "mini-platform," as it were. Guitar Hero, for instance, exploded in popularity, and did so through excellent design, fun, and its own unique controller and interface--but it did so on top of others' platforms. It's a lot easier to design an amazing GAME--or series of games--than build a platform upon which for games to rest. (Let alone get everyone else interested in your vision for it.)