Offsides like that are very rarely going to happen, besides football is based around marginal calls which people are more willing to accept so long as it doesn't take 5 odd minutes to manually draw lines on a pitch, and that then still potentially be wrong.
Semi-Automated offsides are going to 100X more accurate then that, and while we can't see in that image that the player is offside, i'm probably bet he actually is, even if it is by a millimeter.
The more I think about it the more the Wenger rule just makes way more sense.