Koeman is a decent manager but he's all flavour of the month like Laudrep, Martinez and even Coyle were at one point.
He had terrible spells at both Valencia and AZ previously so I imagine it won't be that long before we see Southampton start going backwards, though they might still make top four given how abject us, Liverpool and Utd have been at times this season.
As for Wenger replacements it can't be anyone other than Klopp. He's tailored made for us - similar philosophy, right age and a dominant personality to take control of the entire club. And he always speaks very highly of us and Wenger.
Eventually he's going to get sick of the lopsided nature of the Bundesliga with Bayern able to poach their best players next summer (with Reus up next) and decide to call it quits. A hefty pay package should also entice him as well as basically the opportunity to go into to go into a club which has the resources to dominate everything.
You won't get the Moyes BS of the players not respecting him either because he's an unknown quantity who hasn't won anything either. Tbh, I'd much rather stick with Wenger until we can be certain of getting Klopp rather than jump the gun for a lesser manager. For all of Wenger's issues, we're hardly in free fall right now - just not where we ideally hope to be.