The case against Rooney's already been laid out, but anyway, since I'm here and repeating yourself is nine tenths of the OMITT law:
He's aging and already in decline, there are question marks over his motivation, he's already won everything, he's a controversy magnet and doubly so if he transfers to us; he's emotionally fragile, a puppet of his agent, and needs to be the centre of attention; he's possibly not worth £30-40m plus £200k/w although who knows in this market, but probably not; and last but not least, he's someone widely hated and despised by Arsenal fans for years who's hardly prepossessing to look at.