Depends on what your expectation is really. If a club of Arsenals size cannot even muster a title challenge beyond the month of February for 10 years running, that is an unmitigated failure. The lesser I talk about our CL shenanigans, the better.. that'll just make my point stronger.
Let me ask you this... have you even once in the last decade felt that we are putting our best foot forward in almost all the games throughout the season? Many, if not most of the fans could see a mile away that we are almost handicapped right at the beginning of the season. Adequate cover for players in suspect positions were never developed or purchased. Every season we would go in depending on injury prone players (Diaby, Rosicky), playing players out of position and being tactically childish.
It is a vicious cycle with our manager. 2 years ago when we were leading the table at the top in February, there were a few on here who said that we will fold and die when we hit a tough run of games. Ditto for what happened this season. Where do you think 'these people' on here have their wisdom from? It is because Wenger tries the same nonsense EVERY season and expects different results. At least if he tried something and failed, I'd acknowledge his efforts but he hasnt done it in 10 years and he wont do it if he is here another 10 years.
So yes, he is a failure in terms of what we could have and should have achieved. Has he completely ruined the club? No, I think with whatever nonsense he has pulled off in the last decade, we are still one decent manager away from being a strong team in England (and possibly Europe) and that is the only thing that makes me sleep at night well. However the concern that almost everyone shares on here right now is that if he continues any longer, he is going to actually damage the club.