The main thing Switch needs to do to beat Wii is get decent support for more than 4 years.
Nintendo dropped Wii like a rock after 2010 to shift their teams to making 3DS and Wii U games, and fucked up on properly localizing and advertising games like Xenoblade, The Last Story, Pandora's Tower, and Fatal Frame 2 Remake, which could have softened the crash.
Switch's third party support is so good that Switch would pass the Wii even if Nintendo gave Switch the Wii treatment.
The DS received only a few big Nintendo titles in its later years (as usual during Nintendo's time of running two consoles concurrently, almost all of the big hits were released in the first half of a console's life), but that didn't prevent the DS from having a prolonged peak because it had a lot of third party support.
I suppose people will keep being puzzled because Switch has a trajectory similar to the DS or the better selling PS consoles, but that's because they subscribe to myths like "DS is an anomaly" and "PS consoles always have long legs" instead of doing the most basic of analyses. There are no inexplicable factors at play, it's as simple as "software sells hardware." Switch has quickly built a library that contains hundreds of quality titles and has an outlook of more and more coming to the platform, that's why it hasn't peaked in year 2 like many of its detractors wanted to believe.