I recall when Apple first introduced the X and everyone was complaining about the notch, someone mentioned that for years the iPhone was immediately identifiable by its face, with the top/bottom bezel, rectangular screen and home button at the center of the bottom bezel. They pointed out that the notch creates a new immediately identifiable face since the home button and huge bezels have disappeared. So it may be about branding as much as anything technical or logistical. If Apple makes a phone that is all screen save for a dot for the camera and a dot for FaceID, how would you know it’s an iPhone looking at just the front of it?
Indeed, I think one of the reasons Apple is different to other companies is that they seem to plan years ahead and they don't waste resources when they don't need to. This planning also accounts for features and when they will be released.
The notch was necessary because they wanted to differentiate the iPhone with FaceID, but couldn't put it under the display. And so they introduced the notch, which at first people didn't exactly love, but it's become part of the identity of the phone and Apple milked it. The notch just screamed iPhone and it's so recognizable that smartphone icons now show a phone with the notch. TV commercials that market apps for various services show an unbranded phone with a notch etc. It's become so synonymous with the iPhone that Apple don't have to rush to remove it.
Which is why I think they are waiting to remove the notch when they come to a point where it will be the differentiating factor. Last year it was the third camera, this year redesign and 5G, next year it will be ProMotion and then probably the year after that they will remove the notch. The days of S updates ended with the release of the X line-up, it's no longer enough to just improve everything under the hood because the phones are so good and can last a long time, so Apple needs to find differentiators every year to market with the new iPhone. Getting rid of the notch will be a big deal and they can't let that go to waste. This is why I am always amused when people are upset at the release of the new iPhone with the same notch. They aren't going to waste that on a refresh that already has selling points. Instead, they'll space it out and make sure every Fall there's something to talk about, because how much can you improve the A14 and why would you need to.
This is my guess. I don't have any actual info on anything, just what I see with other aspects of the Apple ecosystem. Apple don't rush to update too many things all at once because otherwise you kill the appeal that the future iterations would have. Seems like smart management and good business to me. Yes, we'd like all of the features to come yesterday, but there's nothing in it for Apple, so... Frankly, it would be stupid to remove the notch now when there are other changes to be sold first.