The Scottish knight has won almost every honour in the game, although he has eschewed national team management except as a brief caretaker manager of Scotland for 10 games in 1985 and '86.
By many young people's standards Sir Alex is now ancient, having celebrated his 71st birthday on New Year's Eve. And yet there would not be a single United supporter of any age who would object to him going on indefinitely as long as he keeps winning.
Last season, United won nothing, and that was a shock to Sir Alex as much as any United fan or the public at large.
We may not know until he publishes his post-United memoirs whether he was going to retire at the end of last season. Losing in the last minute of the season hurt him deeply, far more even than any of his players. Now he has a new challenge: to knock his "noisy neighbours off their perch.
Some might say that is where he derived an apparent new burst of energy from. But that is to deny the evidence that is right in front of our eyes every match and week in week out.
This is a man doing what he regards as the best job in the world and which he gets paid handsomely. He loves every day of it. He rises with passion and sleeps with a smile on his face.
So why would he ever want to give that up?
But one day he will. As we've indicated previously, that day may be approaching although there should be no doubt that his retirement is entirely in his own hands.
There are increasing signs that he may step down (or up, into the board room) at the end of this season or next season at the very latest. It may well come down to whether United win the Premier League this season and whether he thinks they can win the Champions League next year at the latest.
And what would he do if he retired? We have suggested that there will always be a job for him "upstairs," whether replacing Sir Bobby Charlton as the wise head in the board room or simply acting as a mentor when needed for whoever succeeds him.
Any man big enough to take over the reins from Sir Alex will need to be very much his own man, but big men are never too big to ask for help when they need it.
That is a sign of that increasingly rare quality in our society: humility. Sir Alex has this quality, despite his apparent arrogance and single-mindness as perceived by some others.
He has achieved much in a truly great career; and he has made mistakes, some of which he has honestly owned up to or acknowledged. He sold Jaap Stam too early for example.
He has also shown himself to be inflexible or intransigent at times and there are signs that his judgement isn't always sound.
Some might think him a dinosaur to still be using the "hairdryer" in a world where the quiet word or the arm round the shoulder seem more appropriate for adults on more than £2 million a year.
So if he decides to continue indefinitely, how could he change in ways that would benefit Manchester United and leave a lasting legacy of respect even among those for whom football is a dirty word?
In the slides that follow it is important to note that these thoughts are not meant as direct or implicit criticism. One of the great qualities of "big men" is that they can receive constructive observations openly and reflect. They may disagree, as is their right, but we can all improve somehow.
Sir Alex is Sir Alex and that's what has made him great.
Life is a never-ending journey of learning and growing. That is the philosophy that drives players like Ryan Giggs, Ronaldo and Chicharito. It is also a characteristic of Roy Hodgson and Nelson Mandela.
The latter is arguably one of the greatest and one of the most humble men of all time. If you don't agree, visit his tiny prison cell on Robben Island.
One of the greatest footballers of all time, Sir Bobby Charlton, also shares that quality.
Manchester United would be fortunate indeed if Sir Alex succeeded Sir Bobby in his role with the club. Together with their shared mentor, Sir Matt Busby, they share great human qualities of leadership and generosity, both in kind and in spirit.