I don't think you realise how much work porting requires. If you look at the amount of ports coming to Wii U Deluxe... I mean Nintendo Switch my bad, then you can certainly say it sacrifices significant time and manpower for new high quality games. Even if you never had a Wii U, just buy one and get all the games without any obvious downgrades and at a much cheaper price and a much bigger library.
Ah yes that dizying mountain of 2 ports from Nintendo and their direct partners is just a diabolically CRIPPLING strain on their resources.
Nintendo has far more new 1st and 2nd party offerings coming to the Switch in its first 10 months than ports. Zelda (crossgen, not the same), ARMS, Splatoon 2, Fire Emblem Warriots, Mario+Rabbids Kingdom battle, Super Mario Odyssey, and Xdnoblade 2 are all retail new first and second party offerings in these first 10 months. And that doesn't include Bomberman, which is a third part exclusuve.
What third parties do is mostly beyond Nintendo's control.