A pundit was saying look at what Wenger has done since he joined, and that it shows he deserves to go when HE wants to. Other than the flawed logic (so if a manager does well for 5 years, he should stay no matter what for the next decade or two?!), let's compare:
20 years before Wenger
First Division: 88/89, 90/91
FA Cup: 78/79, 92/93
League Cup: 86/87, 92/93
Euro Cup Winner's Cup: 93/94
General league position: Around 4th
General European performance: First rounds of UEFA Cup
Wenger's 20 years
Premier League: 97/98, 01/02, 03/04
FA Cup: 97/98, 01/02, 02/03, 04/05, 13/14, 14/15
General league position: 4th with the odd 3rd
General European performance: First knockout round of Champions League
Sounds like all he's done better is win the F.A. a couple more times after it stopped meaning anything. Certainly the last decade makes dire reading. It's easy to forget how many points behind the other teams Arsenal finish every season. It's a lot.