I think there could be more managers who could have pulled off a long term stay at a club with ongoing success. I just don't think clubs allow them the luxury these days. Mourinho, had he never left Chelsea, would have been a fantastic example. But they "parted ways", rather stupidly, and I do feel both sides regretted that such a thing came to happen. Likewise, Guardiola should never have been put under pressure at Barcelona - they shot themselves in the foot big time by doing so. If they had made him feel like he was king forever, he might have stayed for a longer time. But then, he strikes me as a manager who doesn't want to stick around at a club for a long time, what with his fascination for one year rolling contracts.
I imagine Arsene might have seen much more success if he were at a club that was willing to pay up in order to keep it's big players. United is a club that should be capable of having a long term manager and getting trophies by keeping it's best players.