The thing people don't seem to realize is IPs not created by Nintendo themselves are rarely worked on by anyone other than that specific dev. The only real exceptions are Pokemon (because the market for that IP far surpasses GF's output capacity) and Kirby (because Epic Yarn originally started as another IP). These are Nintendo's first and second biggest IPs not created by them. And Kirby only happened once! I'm sure Retro or Monolith could do GS justice, they're both quality developers, but thinking they might actually take the reigns on a c-list non-Nintendo-created IP of currently limited market appeal is grasping for straws.
Yeah, Retro's name is retro, but, and not to say JRPGs aren't in their wheelhouse, because they've had a limited wheelhouse, but the correlation between those two is tenuous at best. Also, GS isn't retro, it had a game this decade. Regardless, having anybody other than Camelot make a GS is frankly random, and not exactly in line with precedent. It'd be one thing if Camelot up and left Nintendo's circle like Rare, but they haven't, they've already made their two token sports games this gen, and GS is their IP. Nintendo's too, sure, but until it regains some commercial success, I see no reason why it wouldn't live and die by Camelot. On top of the fact that it's not how Nintendo operates, there's really not a whole lot of incentive for Nintendo to make another team work on it, especially an A-list team like Retro when they don't even know if it will provide a safe return on their investment.
In short, GS is not in the Mario, Zelda, Metroid, DK, StarFox, F-Zero boat where Nintendo created it and will lend the IP to other teams, it's in the FE, AW, Kirby, Xenoblade, Custom Robo, Chibi-Robo, Starfy, S&P boat where the IP is owned by Nintendo but unless there are some real extenuating circumstances, it doesn't leave the developer who made it. And I'm not saying there can't be exceptions, just that I see no reason why there would be for a series like GS given its current state.
Also I don't think there's ever been a period in time where what every single Nintendo team and affiliate is up to has been public knowledge and I doubt that will change any time soon. I mean Nintendo itself has like... 10 or more dev teams, another half-dozen first party studios, and like 15 second-party partners.