Pokemon-Games are not from Nintendo itself. They own shares of the franchise, but it seems they are overall not directly involved into the game-development, unlike with Zelda. And Pokemon in general has different problems regarding quality. They are more time-constrained, stressed, and seem to have some internal struggle in the last years. While Zelda seems to had the liberty to develop peacefully for years on their own.
Not even close. It's a BOTW expansion pack writ large, there's little performance or graphical difference between TOTK and BOTW, a game that released six years ago.
This is apparently incorrect. I'm avoiding detailed tech reviews right now so as to not spoil myself, but reports are that the original BOTW held itself back in order to accommodate the Wii U. Draw distances appear to be higher, objects are more detailed and there are more of them, there are more LODs, and the framerate is now reliably 30 FPS in all but the most demanding scenes (which sounds like faint praise, but if you've ever played the original, is a definite improvement).
It helps that the original game was utterly gorgeous, thanks to inspired art direction.
Same engine, slight retune, not six years-worth-of-dev-new. Much more of an iterative upgrade. Compare, for instance, Ocarina of Time (1998) and, Windwaker (2003), vs Ocarina (1998) and Majora's Mask (2000), and you'll see that BOTW/TOTK is much closer to the latter than the former.
