Fergie was, Wenger was. Klopp and guardiola have had decent spells. I get what you’re saying though, a manager can only last as good as they do kind of thing. But when I say long term manager I mean someone that wants to create a style and environment that everyone is happy working in. Not managers such as Tuchel, Mourinho, Conte who love to cause chaos and upset everyone.
Fergie and Wenger were generational managers that joined directionless clubs and implemented systems from the ground up, Klopp did that at Liverpool also, City did that for Pep. These managers aren't just knocking around waiting to be picked up, they're at top clubs already, or clubs where they're now unsackable because they've already implemented this kind of support system.
A manager becomes long term when he wins things consistently, you don't appoint someone with the aim of them being long term, that's doing things the wrong way around. Look at Ten Hag, does he deserve 5-10 years before we judge him? In the mean time we could be missing out on the next Pep or Klopp because we're persevering with someone who isn't up to it because we want a 'long term' appointment.
The club should create the environment and style, then you hire managers who fit that mould so it's seamless between them. We can't have our whole club's fortunes down to one person's ability at running absolutely everything at the football club.
Well yeah, the 3 managers you mentioned are all dickheads so if you stay clear of those types of characters then you won't have many issues.