Stop comparing Wenger to SAF. They're light years apart. In the last 9 years Wenger has won as much at Arsenal as Moyes did at Everton... and he was a piece of pi55 to replace.
I think someone did a Moneyball type statistical analysis of the two managers a few years back and (if I remember it correctly) it came out that SAF was worth on average 14 points a season, over where his side should be based on how much they paid the players and how much they spent on them. Wenger came out at about 11 points a season. No other manager in England was above 3, but Moyes was one of them that was around that.
I think what Manchester United fans (and Arsenal) have to come to terms with is that managers that contribute so much to their teams simply don't seem to exist outside of these two.
United in particular, because they are used to winning things are going to be in serious difficulty. Many of us have looked at United teams (in the last few years particularly) and wondered just how the hell he managed to win games with players that were as bad as we thought the United team were. Wenger is the same, some players leave Arsenal and just fade away to nothing. Song, Hleb, Flamini, these players just don't look the same when they leave. Our fans are constantly saying our team are crap!
I really think United are 100% daft sacking Moyes, joking aside. SAF needed years to perfect his style and they need to train a manager if they hope to rival Chelsea and City in the future. There is no way that anyone is going to learn how to consistently beat the top teams at a club where they have no chance of consistently beating them. Learning how to nick a result by packing a defence and trying to get lucky with a long ball forward isn't going to cut it.
I think when Wenger leaves, unless we are very lucky, we are facing 3-5 years of mid-table performance, unless the new manager inherits a star team, and possibly more if we keep chopping and changing. I think United face the same thing.
Learning how to manage a team playing aggressive attacking football that can reliably compete with the big teams and reliably beat the smaller teams, is not something that you can learn with teams that don't give you that environment. Any new manager coming in, unless they are very lucky, or like I say, inherit a great squad, has the odds dead against him succeeding.