Why should Wenger be sacked or resign from Arsenal?

I am an English football enthusiast and I have believed for years that Arsenal were a good example of how to manage a team: they perfectly balanced expenses and trophies.

However in the last years things has changed and Arsenal spend a fortune and win very little. Why? There are many reasons.

First of all it is fair to say that probably Wenger has not been lucky with his investments: Özil is not the player who can solve every match and so is Sanchez.

But who is to blame for these uncertain investments? Obviously the French manager, who could have signed different players, for example Fábregas.

Furthermore I believe that two aspects of these last years of Wenger's management are clearly unsuccessful: the attitude and the defense.

How can a team which yearn for the title start all the matches against big teams with a wrong approach? In football and also in life attitude is everything and without it you can't be a winner.

And the defense's approach too is not the one we should expect from a big team like Arsenal. Wenger wants his defenders to be aggressive because this is his view of football, but I believe that it's not applicable if your defenders are Mertesacker and Koscielny.

Therefore I think Wenger's reign should be over; it's not easy to leave such a successful manager but a big team must know when it's time to turn the page.