Obviously MP4 was a no show at E3 and we found out last year the whole game has been started over from scratch. With the recent tweet from Retro that they are just now looking to hire for a key art director role in the game, it would seem likely they still haven't even started official development yet. Moreover, with the last five years amounting to literally nothing over at Retro, we can't assume they are at all capable of developing a top notch game in a speedy manner (if they're even capable of developing a top notch game at all these days, but that's for a different topic).
In other words, MP4 is still a long, long ways away. Like maybe up to 5 years long, but 3 at the very least. That range would put it in the same proximity as when the Switch successor will likely launch (2022 at the earliest, 2024 at the latest). It seems like a given at this point that at the very least MP4 would be another TP/BOTW dual system release. However, those games were well into development for current gen consoles when they got ported over to the next gen ones. Given it seems Retro hasn't even started development on MP4 yet, I think Nintendo may very well want the game moved to the Switch successor. While I know some might say this would create a backlash since it was originally announced as a Switch title, keep in mind that the game originally announced no longer exists. It was scrapped, everything is starting over.
Furthermore, the Switch is no Wii U situation. Nintendo had to deliver BOTW to Wii U owners given the shortage of truly AAA great games for that system. Switch on the other hand is already looking like it may amass one of the greatest software libraries of any Nintendo system ever when all is said and done. Anyone who bought a Switch based off of the 2017 MP4 announcement would be highly unlikely not to be satisfied with their purchase of the system overall because of one game not coming.
So in the end, given all these factors I think it makes a lot of sense that MP4 will be commissioned as the showcase title for the Switch successor's launch window. The Switch's place as one of the most successful consoles in history is set in stone at this point, Nintendo doesn't need MP4 for the Switch anymore. What better way to create hype for the successor than a truly next gen Metroid game with years of built up anticipation?