So I agree with your first paragraph, I think I'm in that same camp. I'm not saying the 'good things take time' argument should be used to keep Ten Hag here specifically, I think any manager requires time. Given the positives that I have seen with ETH I would like to wait till the end of the season where I hope he can manage at least 5th, but no excuses going into his third season (Where the club can sack him even a few games into the season after sounding out a few managers like Enrique, or Nagelsmann in the summer) - He's levels above Moyes for me, and he's above Ole in that we saw a more controlled style of play last season. He's not above Mou and LVG for sure because of those managers' pedigrees, but we specifically wanted a manager that was up and coming that would work with youth, would play an attractive style of football and was looking for a long term project - ETH was the best pick at that time, certainly over Pochettino who is having a worse season at Chelsea with more money spent.
I don't think there is a large enough pool of historically great or currently high-performing managers to pick from at the end of this summer either - Ange while he's having a good season with Spurs is still unproven for me personally and I'd rather make my conclusions after his second season, Emery is having a great season with Villa but didn't find success with Arsenal so we'll have to wait and see what his Villa side do next season. Xabi [Irrelevant point - unless we are talking about F1] is doing well of course, but again not proven enough - He could go to Liverpool and still do well there, but he will do well there mainly because of the legacy that Klopp and FSG have built over 9 years, in addition to his coaching prowess (He could still flop there of course). Ancelotti is taken up with Madrid, and Zidane is not interested in United, Tuchel taken up with Bayern. Enrique maybe is an option, but might not be interested. I just don't see many managers improving this team in the short term (Personally don't see any bar Klopp or Guardiola from the current Prem batch).
Without injuries, we are about 3 starters and at least 4-5 squad options off from getting close to City, Liverpool and Arsenal currently. We should've been 4th this season comfortably, but it is also no surprise (while disappointing) that we are 6th when mostly playing with sub-par squad players who themselves have needed replacing for some time.
Without having to be an ETH fanboy, I think ETH will have his pick of clubs (Not quite the RMs and Barca's right away but very good clubs) even if he gets sacked at United. I just worry for United who will be left neither better nor worse off with the next big up and coming manager, if that is the only change we will see at the club.
P.S. - Why is it that I can't post the name "[Irrelevant point - unless we are talking about F1]" without it being it being referred to Fernando?