The Wii U was a known failure though, a sinking ship, so of course they were gonna save what they had for LM3 for Switch.
Far as Prime 4, I'm not really worried. The Switch was already established to be supported longer than the average system cycle by Kimishima; extra longer in this case thanks to COVID, which is in a great position with the extra sales the system and its games have been getting from the covid downtime. Remember too, this isn't like previous Nintendo gens where their console also had a handheld to fall back on (the Wii U & 3DS, Wii & DS, etc), with the Switch filling both niches, so it'll be around and supported longer just on that. I see MP4 coming out at the tail end of the Switch's lifetime at worst.
The Duke Nukem Forever comparison is overrated. MP4's situation is more like Doom 2016, which also had development restarts for the better and still turned out great, despite being in development hell for awhile. The gap between Doom 3 and Doom 4/2016 honestly isn't dissimilar to that between MP3 and 4.
And hiring other people mid development is not uncommon. The fact we've been getting somewhat consistent news of these Retro MP4 hirings before and after the start of COVID is honestly a good sign. What intrigued me about the last hiring from yesterday though, is that Dylan Jobe was also involved in Doom 2016 as it were, who is now apparently director of development for MP4.
Game's gonna be fine. They just announced MP4's existence too early to shut up fans, which became consequential due to the dev restart. Things like this happen probably more than you know with games we get, but with Metroid they had to be more transparent after the failure of its last games.
Other than that, still hoping against all odds as to an eventual Prime Trilogy Switch port, whether it currently exists or not. Now that we got Mario Galaxy on Switch (technically, with the Mario 35th collection), I feel they could do a workaround for Prime 3 far as the Wii motion controls, in making it adjusted to play on joycons. Unlikely to get a Metroid 35th anniversary collection though when Zelda exists, but maybe MPT ported in some form can still happen at some point with that in mind. The thought of Metroid getting nothing for ports/remasters leading up to Prime 4 is inconceivable to me, but Nintendo be Nintendo sometimes.