Good point, I never thought of the idea that maybe MP4 was going well, but they just felt the Switch wasn't up to handling their vision for it so they are restarting with more powerful hardware. However, I'm not sure I can agree on the idea of a mid gen Switch upgrade that plays exclusive games the Switch doesn't. I mean, that would be pretty hard for Nintendo to successfully market and sell. One of the biggest reasons listed for Wii U's failure is that the name made it sound like an upgrade not a new system, and most people aren't going to want to buy an upgrade of a system they already have to play new games. Upgrades of systems work best as a way for tech junkies to get their fix, not by being required to play certain games.
Have you already tried to run Ocarina of Time N64 at 60fps on an emulator, on a machine 1000 times more powerfull than a N64 ? You can't, because the game has native script limitation which just prevent it. BotW is certainly more a WiiU port to the Switch than a native Switch structure downgraded and ported to the WiiU. I totaly think they can decide to restart, like 80% of the work to rebuild the game optimaly for the Switch Pro, and then downgrad it for the base switch.
In that case it is the same thing to announce : "we restart from the begining" or : "we have to rebuild most of the game structure".
I am very superficial in hardware/software technic, so anyone here can say that I am saying bullshit, I try to stay general only as much as possible.
Current PB on Secret of Mana remake : 2h27 (2nd)
Strongest worldwide achievement on TGM : 1st European S13 rank
WR on TGM alternative mode : Fastest ITEM S9 rank ( <-- Just got massacred by the 2nd most powerfull player in the world )
Current PB on Power Ranger (Game Gear) : 10min06 (World Record)
Non-geek activity : ThermalHungary