Sir Alex's longevity comes from the fact that he was not fanatical about a particular tactical system.
A lot of managers achieve success doing things a particular way. However, often they find they can't just keep doing the same thing and get the same results. Either they lose the players that made it work, or other coaches copy them or other coaches figure out ways to stop them. At this point coaches who are wedded to their ideas find it hard to keep winning. They believe so strongly in their 'style' that its hard for them to accept football has moved on and they must do too.
Ferguson was clever enough to read changes as they happened and open minded enough not to just stick to what worked before. When 15 years ago fans were slagging off Carlos Quieroz and demanding the return of 4-4-2, Ferguson had the vision to be able to change. That's what made him so great.
I do not agree that Fergie's football followed his players. Ferguson was ruthless at getting rid of players who no longer suited his ideal of what he wanted to achieve with the team. For example, the clear out that introduced the class of '92 wasn't just a change in personnel. It was a change in style too. There was nobody doing the Mark Hughes or Andrei Kanchelskis role at United after that point, plus the midfield configuration gradually changed as Nicky Butt was eased out by Paul Scholes. Likewise, we won the Treble playing 4-4-2 but Fergie opted to ditch it and rely on a focal point in attack, Ruud Van Nistlerooy, because he saw how European football was developing. Then when a new wind of change, led by Mourinho, came in Fergie showed Ruud the door.
Sir Alex proactively shaped his squad to achieve what he knew he needed to do to win. That was his genius. He, better than any coach in the past 30 years, could figure out what was coming next and adapt to it. IMO had we given Sir Alex the kind of funds we gave his successors, especially Van Gaal and Mourinho, he would have reshaped the squad to deal with tiki taka too. Its hard however when you're expected to replace Ronaldo with Obertan.