I've never bought into this mindset.
Why can't it be both?
Why can't a manager genuinely enjoy being at a club at a particular moment in time and really buy into the culture and the environment, but also have one eye on furthering his own career?
I've never believed that the likes of Roberto, Brendan, Potter, Martin etc didn't love the club while they were here - I believe they did, unfortunately for us they just got better offers that they'd all have been insane to turn down.
Coaches don't think about a club like a fan does. Simple as.