Coldplay is the biggest alternative rock band of this generation, but I don't think they'll be legendary. I love them, but they're basically U2 - 20 years. It's like a Michael Jackson/Usher situation.
They didn't push any boundaries. They just did Radiohead-lite and U2-lite for their whole career. Legends have to bring something NEW to the table. And I love Coldplay, but they've done NOTHING groundbreaking.