Logical fallacy bonanza in your first paragraph there. No, it doesn't mean that every other manager is destined to fail. But it does mean that you don't have a basis for assuming another manager would succeed, just because he has in the past. "There’s plenty of basis for doubting whether Ole can get us back to the top." Well, the only basis you've presented so far is writing "there’s plenty of basis for doubting whether Ole can get us back to the top." Secondly, you're applying standards you're not applying to the alternatives. Certainly it's uncertain if Ole can get us back to the top. But it would also be uncertain if it was Nagelsmann or Tuchel or Pocchetino. You don't know, I don't know, nobody knows what OGS' ceiling as a manager is. The only thing we have to go on is results and how things are shaping up. None of which indicates that he has reached that ceiling yet.
So that leaves the question on what basis you can really argue that we'd have done better, or will do better, with another manager? As far as I can see, you don't have one. Of course, it's possible to have a strong opinion anyway. But is it smart?
We ARE much closer to a title challenge now than we were in December 2018. Hugely closer. And anyone who thought 6 months before that we were remotely close to a title challenge were brutally proven wrong by the ensuing months.
I for one would never have expected in December 2018 that we'd be this close to a title challenge now. I assumed then we needed a bigger demolition job and more time, and would have to soak up more suffering. If on the other hand someone had asked me in 12/18 how I would feel about finishing 3rd and 2nd the next two seasons, I'd have been thrilled. Not because I'm happy with top 4, but because I want a proper contender built, and accept that takes time.
"Is this good though?", you ask. And answer "not really". Why? Your answer to that really boils down to one thing: Because you'd have liked things to be better. They're ok, but they're not really good enough for you. Only slightly improved from last season. Bla bla outside factors, don't mean anything because who knows when we're back to normal. Some things are good, but that just means the results should have been even better. Meh, meh, meh.
Which, I'm sorry to say, is just arbitrary. You come across as someone who's just made his mind up that things aren't that good, or, where it cannot be denied that they are sort of good, should have been even better, and who simply isn't going to be pleased.
Your last point is just arguing against an absurd strawman. I really don't know why you imagine anyone would believe that further progress is guaranteed. So that's one gapingly open door kicked in.