Maybe I'm just a bit too optimistic when it comes to Retro studios, but it seems like a lot of the skepticism around them is really undeserved. It would be one thing if the reaction to them was neutral or more fair, but more often then not as with this prediction, when someone is critical of Retro they take the whole mile instead of the inch. There were a lot of negative things said about the studio that made no sense but were believed until they got put on the job for Metroid Prime 4. Remember when people thought they were going to be shut down because of reviews from a site where anyone can review any company, even if they never worked there? Remember when people used the criticism that they didn't even have anyone from the original Prime days, but then it was shown that like 15 of their top brass is from the Prime trilogy, and a few others were from the early DKC days? Yeah, maybe I shouldn't dwell on that so much, but that really makes me take all of this Retro cynicism with a big grain of salt. Five years isn't a lot of time for one of Nintendo's smaller developers (the last time we got numbers they were only above 70), especially one that's known for utilizing the hardware of Nintendo systems to great affect. Five years is basically the length of one game for most Western developers.
As already pointed out, a lot of the stuff you use to justify your prediction don't make sense. Nowadays multiple art directors might be needed, or maybe the art director they had left. How does that indicate they haven't started development? We've heard rumors that they showed a demo off to Nintendo to try and get the project, and while rumors can be extremely inaccurate (see the ones I mentioned earlier), it makes a lot of sense considering Nintendo handed them a really beloved IP. Even if the demo was just that; a demo, with it's contents being scrapped, they would at least have a basis of reference and would probably be further in development.
The worst argument by far though is where you basically admit that Nintendo doesn't need Metroid Prime 4, but then are skeptical of Retro's ability to deliver just because of the past 5 years. That might not seem like a contradiction, but think about what's being implied there. Metroid Prime 4 was never a title Nintendo "needed" to do, Metroid isn't a big console seller. If Nintendo didn't have legitimate faith in Retro, they would have just cancelled the project outright. Even in early 2019, they never needed Metroid Prime 4 because the Switch already justified it's existence with a laundry list of high quality exclusives that, unlike Metroid, actually sold a significant amount of systems. So why would the game even be entrusted to Retro unless they had the actual capability to deliver?
Now, if you notice I haven't tackled the issue of release date and that's because you could very well be right. I don't think so but, it could definitely happen. That I don't really care about. As long as it ends up being a great game, I don't really care. I will say though, I find 2024 completely unlikely. Even though I think Nintendo could make Metroid a much more popular IP with Prime 4, being so optimistic as to give it till 2024 to be developed seems too unlikely. It would either have to be an extraordinarily expansive game, or one that Nintendo saw as a huge potential cashcow, and I don't think neither will be true.