He would have been sacked already at most big clubs in Europe. It's a long-term legacy of Ferguson's unique story of escaping the sack and setting up a dynasty, and then Ferguson saying you must back the manager when he left, that United are very patient with managers. But every case is different, there is no guarantee it will get any better with patience.
It's got feck all to do with Ferguson's story.
We've not given a single manager post-Fergie even three full seasons, and we're less than one and a half seasons into Ten Hag's tenure and he looks like very much like he'll be getting the boot this month.
Moyes didn't manage one full season. van Gaal got two, but was booted for failing to qualify for the Champions League, despite having won us the FA Cup and qualified for the CL in his first season. Mourinho and Solskjaer both got two, and were both sacked less than halfway through their third, despite having won the EFL Cup and Europa League, then managing a 2nd place finish, and managing the first successive top four finishes (3rd and 2nd) since Ferguson retired, respectively. Ten Hag finished 3rd and won us the EFL Cup, and despite how shite we've been this season, has still managed to win enough points to have us sat 6th, still within touching distance of the CL places.
You could probably count on the fingers of one hand the number of fans that think any of the previous managers should have been given more time, and you'd struggle to find many with any strong opposition to the suggestion that Ten Hag gets sacked now.
The issue with the fans isn't there there's loads of them thinking every manager could be Ferguson with enough time, it's that there's loads of completely deluded fans who hold the idea that, 10 years on from Ferguson retiring, United are in a remotely comparable state to clubs like Real Madrid.
One has enjoyed two EFL Cup wins, an FA Cup win, and a Europa League win in that time, with their best league finishes being two in a distant 2nd and two in 3rd, and the best they've managed in the Champions League being two appearances in the quarter final. The other has won the Champions League five times, three league titles, and two Copa del Reys in the same time frame.
Of course Real Madrid would sack a manager when they're sat in 6th, more than a few games into the season, because they're a far better team than Manchester United, and have only been in that situation once in the past 10 years. United have basically been in this position in 8 or 9 of the last 10 seasons. Sometimes it's got better, sometimes it hasn't.
However, if we shit ourselves and hit the reset button every time we have a rough patch, we're going to continue shitting ourselves and hitting that reset button again and again over the next 10 years, just as we have over the past 10. There's a massive middle-ground between giving someone a bit of time to get over a hump, and giving them six years just because. No one argues for the latter.