I wouldn't say "going back to their roots" per se, maybe more like "going back to the mid-late 1990s".
With Switch I think they are letting go of the Wii/Wii U era and going back a little bit more to the N64/GameCube era with things like more big, open-world type titles. Super Mario Odyessy is going back to the open world style of Super Mario Sunshine (hopefully it's a better game though).
Metroid Prime 4 is another nod to that audience. I'd say Nintendo's philosophy with Switch is to take their top IP and make them big/epic/pleasing to want the fans want, whereas with Wii/Wii U it was to try to simplify said franchises to try and cash in on casuals.
Mario is the clearest example of that ... Mario 3D World was trying to make 3D Mario easier to play and bridge to the 2D series, Mario Odyessy breaks completely from that and goes back to the Mario 64/Sunshine roots of 3D Mario.
Now that Nintendo is not interested primarily in expanding the gaming population (due to smartphones allowing everyone to play), they have shifted over to making sure their popular IP are giving Nintendo fans the scope of experience they want.
