The sheer number of games in which we've been disastrously awful without a real striker, then Giroud come on and how we play has been completely different. We've had 3-4 games playing this Sanchez/Walcott upfront combination in which we've deserved to be beaten soundly with absolutely awful attacking performances only for dodgy penalties, dodgy calls, handball goals to make Wenger think it somehow worked.
I've also said for years that Giroud isn't very good but would be a great sub and alternative/two striker option for late in games.
The problem is we need someone better than Giroud to start ahead of him, not Sanchez, who is as far from a central/lone striker as I think Arsenal have ever had during Wenger's time here. His consistent decision to play Sanchez should have cost us 10+ points by now, but even with the late game flukes has cost us points in both the league and CL.