I was confident about Mario Kart and Smash Bros. transitioning over to the [Nintendo Switch] since E3. It was just extremely logical. I also added Splatoon to that idea of thinking, but additionally I want to posit this.
Splatoon was very successful. However, it was successful under the restraints of an unsuccessful platform and very limited consumer base. Now, Nintendo wants this franchise to stand along their big multiplayer IPs (Smash, Mario Kart). The Nintendo Switch is an opportunity to go for even bigger numbers. Not only would it not make sense to let all the Splatoon content die on the Wii U, but the idea of introducing a title named Splatoon 2 for the first time to an audience, is a marketing disaster. It really makes sense to take those big 3 multiplayer IPs, and build on them, release them as strong updates early in the Nintendo Switch life cycle to a potentially BIG audience.
Meanwhile, the rest of Nintendo's R&D can actually focus on delivering a mix of BIG new entries, and original titles. But it doesn't make sense to let three of their biggest multiplayer games, vanish on the smallest user base ever.