I knew you in particular would disagree with my hypothesis! :P
Anyways, regarding the Wii not being a continuation of the GameCube is not entirely true. While they play very differently, the Wii concept was still originally going to be a GameCube peripheral. I've heard as a knock against the Wii's power that internally the Wii is "2 Gamecube's strapped together". And then there is the fact that when you open the top of the Wii, you find a GameCube smiling back at you:
The Wii was very much an extension of the GameCube. They just managed to hide it enough, so that it wouldn't have a tainted perception associated with it from the start. It could have been called GameCube 2. Then Wii U could have been GameCube 3, or would they have mucked it up anyway and called it GameCube U? *shudders*
Nintendo did say that Revolution would be an entirely new proposal that will use GC architecture at its core, so backwards compatibility is about the only thing where Wii was a continuation of the GameCube. There's also a huge difference between a peripheral and a standard controller; had the GC been a success, Wii would have never happened.
When you have a roadmap, it's about refinement and smooth transitions from one product to the next. In the line GC - Wii - Wii U - Switch, there are significant changes every time. GC failure led to a greatly different approach. Wii success encouraged Nintendo to go back to their original plans by slapping Wii branding on the continuation of the GC. Wii U failure made Nintendo do something different. This isn't a history of small changes each generation leading to greatly different outcomes; it's a history of huge changes having huge impact.
Agreed, though the thread was a good read.
I think the Wii U was always originally supposed to be like the Switch, but because it was too expensive and harsh on battery life at the time, they nerfed to what we know now. So in that way, maybe they've had this roadmap planned since the early 2010's, though that doesn't mean they expected the Wii U to fail, never mind as hard as it did - but I feel in no way they've been planning this for over 15 years.
Wii U was supposed to have separate Gamepads being sold in order to replicate the GC-GBA connectivity where each player has their own screen, so no, Wii U is not a nerfed version of Switch. People tend to believe that it is because of off-TV play, but off-TV play is merely an additional feature of the main purpose of the controller. Because the Gamepad was so expensive, Nintendo planned to launch separately sold Gamepads later, but the installed base grew at such a dismal rate that this plan was canned; Nintendo already had too much trouble to convince the market that having one Gamepad is a good thing.
It's something that many people forgot or never knew about to begin with, but Wii U was supposed to have more than two screens in its final envisioned state. Asymmetrical multiplayer and additional information on a second screen in single-player were the core concept of Wii U, matching up with the ideas of the GC-GBA connectivity. These things aren't even present on Switch.