I don't think it's the same situation. The team was in a meltdown almost purely because of the manager at that time, so Ole coming in was able to quickly refresh everything. A lot of the key players were also young and still seemed hungry.
There seems to be a lot more wrong now than just unhappiness with the manager, so Rangnick wasn't able to get such an easy new manager bounce (he still did get one, just not to the same extreme). A lot of those same players are still here now and they are in the middle (or later) of their career and seem completely set in their ways, not willing to go out of their comfort zone. Our decline in results over the last two months has also coincided with a very significant injury list which I think people don't take into account. Shaw, Varane, Lindelof, Fred, Mctominay, Pogba, Ronaldo, Cavani have all had periods out, while we loaned Martial out and then immediately lost Greenwood as well.
Put it this way, the situation was so bad that I'm not sure that even Conte (who has a decent claim to be one of the top 5 managers in the world) would have been able to get us top 4. Obviously he would have had a better chance than Rangnick did and I don't think things would have gotten quite so bad, but it's very debatable one way or the other. And then obviously Conte would have wanted the job fulltime, and we would have continued another couple of seasons of defensive boring football with a toxic collapse (at least between Conte and the board) at the end of it. There's no way to know for sure yet, but the hope is that Rangnick will be the best long-term appointment. If it turns out we don't really listen to any of the recommendations that he makes then yes, it was a poor appointment. But hopefully that won't be the case.