Wii U's failure had absolutely zero to do with "starting from zero", it had everything to do with plain bad design and marketing decisions.
A cycle where consumers only get 3 years of decent support for their device is not good for consumers, it is terrible for them.
And what gauruntee do you have that a "Switch 2" doesn't have the right design and doesn't have marketing missteps. Zero. The Wii U's failure really was more rooted in consumers growing tired of mini-game-athons that drove Wii to super high sales, you release Nintendo Land 3 years earlier and it sells 20 million, three years later you get treated like crap. Consumers are fickle, you can't bank your company on every console transition going well, Nintendo fails about 50% of the time when they've had hardware transitions.
I'm not taking that chance if I'm the new Nintendo president, no way.
It wouldn't have to be 3 years of support either, it would be more like 5-6 years of support, there would still be hundreds if not thousands of games compatible with the OG Switch and probably all of the important Nintendo 1st party games can scale very easily because Nintendo doesn't really aim for realism in their presentation, so it would be straight forward to have say Breath of the Wild 2 (hypothetical) run at 60 fps 1440p (4KTV) on Switch Pro docked versus say 720p on the OG Switch.
Past that, adopting this model, Nintendo would sell more hardware. Fact is you sell more hardware with more options for the consumer, and significant more options, there's a reason why Apple has like 6 different iPhone models and 6 different iPad models. Consumers *like* the choice, I'm sure there probably is a small group of people who complain/are angry that there are so many iPhones/iPads, but you can't run your damn business listening to this tiny sliver of the marketplace. The fact is it's 2018, not 1980 something, people buy electronics with the understanding that there will be multiple model revisions.