At 26 years old, Mahrez is heading into his peak. Signing him now shuffles down the order of contention for the wide-right players we currently have in Theo, Alex and Welb. He'll be a clear upgrade on all of them regardless of his performances in the league last season. Instant impact.
I'm fine with Wenger buying offensive players again simply to try and 'outscore' everyone else and I'd be rather confident that if we sign Mahrez, he'll be netting 10+ league goals next season. Easy.