Fair point, they're not really comparable.
But did Radiohead even bother to "challenge the mainstream" at all with Kid A? Rather, and for better or for worse, Radiohead avoided the mainstream (no singles etc.) entirely. Pretty much all sales of Kid A could be pretty much attributed to the fact that Radiohead had a solid fan base after Bends and OKC, and i would have indeed been polarising.
But whereas Radiohead have completely rejected the mainstream, I'd still argue, in comparison with most other massive global artists who write their own songs (Matchbox 20, Foo Fighters, Bon Jovi, Oasis, AC/DC), Coldplay's comparative willingness to try something new is quite profound and unique, and it might be hard to gauge from our ears (we are in Bang & Clatter, don't forget), but to the core of the mainstream, songs like Death and All His Friends, Yes and Talk are gonna sound kinda wacky and out there.
So in a sense, Coldplay have challenged the mainstream more than Radiohead in that Radiohead never actually challenged the mainstream in the first place (or second place, depending on how you view OKC), but instead removed themselves from it.
All depends on the definition of mainstream ultimately anyway...