I think a lot more than half will require the nunchuck. Even Wii Sports, Rayman and Wario Ware require it. Racing games and maybe Wii Play are the only ones that don't require it so far. Well, that and some VC games.
I don't see Sony copying Nintendo as a big threat this generation. Wii is in a totally different class and it already has mindshare (and a bit of marketshare) among non-gamers. If Sony jumps on this immediately, then Nintendo still has a huge price difference and the arguably superior virtual console in its favour. Not to mention the Wii Channel interface and it's upcoming (hurry up, Nintendo!) features which I feel are very focused on non-gamers. Last but not least, it's cheaper to develop for, and that's not going to change unless Sony severely relaxes the requirements for PS3 games.
If Sony doesn't jump on this immediately, (like within 6-8 months) then Wii will already have a superior lineup of games as well as a big reputation for offering those types of games - on top of all the other advantages I just listed.