2D Mario is big, but is it really THAT big? Especially so late in the Switch's life cycle? Honestly, I don't think we have a good precedent to follow. 2D Mario games are almost always released within about three years of a platform's launch (the NES, SNES, GB, GBA, DS, Wii, 3DS, and Wii U all follow this trend).
Wonder will easily pass 20 million, but 50 million sales implies years of legs that the Switch simply doesn't have in it.