Arsenal have spent a lot of money in the last few seasons to build their squad but they have not been able to win any major titles and that is clearly a failure. If you look at the game graph, The Gunners have always been able to improve but mentality is a big obstacle that makes performance inconsistent. And in my opinion if this season fails again to achieve achievements then Arteta's position deserves to be evaluated and it is very possible that a change of coach is a must for the sake of the big target to be achieved.
You're so right, since the inception of Areteta he has spent almost £800 million or more than in the transfer market as Arsenal manager. So far no tangible result or outcome to back up this kind of spendings. I still can't understand how a team will spend so much money and still can't give joy to their fans. Arsenal is like a seasonal film where a new season is always expected every year.