In my book thinking that any other PL manager could do a better job isn't close to rational - the vast majority wouldn't have the dressing room for more than a few minutes at best, and the pressure of the United job would crush them. Rangnick was highly respected, and he lasted 45 minutes before the squad discarded his approach.
But even if that were the case, those managers aren't gettable. Emery isn't gettable, at least not now (possibly at the end of the season, although there are lots of candidates I'd have above him for that). McKenna is probably feasible, if expensive, to get immediately, but he wouldn't come as a caretaker so it would have to be a permanent appointment, and I'd suggest there are huge question marks over whether he could cope with the United job. But I'm open to being convinced - what is it about his CV that suggests to you he's ready for the job?
As for the so what, I think you're underestimating the damage that a bad managerial appointment can do. Once we've sacked Ten Hag, will you see his tenure as no harm done? I highly doubt it, and if I'm right then by your own logic "so what, sack him and move on" is a dangerous approach. For sure it's one I don't agree with, I'd much rather us do some succession planning and bring in managers we expect to succeed, rather than setting the bar as low as "worth a shot".
The bit in bold is, once again, a deliberate misrepresentation of my point. It seems to be a common theme with posters who want the manager gone immediately, a rather tedious refusal to discern between "literally nobody better" and "nobody immediately attainable that is worth not waiting for the summer when we can bring in a top manager", which is much closer to my position.
As to your point that our knowledge base is limited in comparison to Dan Ashworth and the other execs, I agree. But those knowledgeable folks have opted to stick with Ten Hag. If you want to defer to their knowledge for choice of manager, well they've chosen the incumbent. Nobody is infallible of course, but you can't make the appeal to authority only when it lines up with your thoughts.