Correct. The club is like Liverpool was in the 90's - too much looking backwards and a lot of fans still take that view.
The idea that you keep a failing manager on because you can't guarantee that the next man in will be an unmitigated success is insane.
There are, at any time, numerous managers who could be the next elite managers, and one or two of them will be. A well run footballing structure (which we now have - "best in class" supposedly), should be able to identify them. It's no different from picking ETH from Ajax. Obviously there are no guarantees the man picked will succeed. If he doesn't, you move him on. The game has changed and managers will rarely stay on for a decade anymore. Stability comes from structure around the club - not the manager. Well run clubs can change a manager every few years and still succeed.
The game hasn't even changed, in all eras managers staying beyond 4 or 5 seasons is a rarity. People base that view on the exceptions, not the norm.
Edit: Also longevity is the product of success not the other way around, it's an other fallacy that people follow when it comes to United.