Well judged in terms of what results in a good end product. Doom on Switch for example is still a great game even without the map editor. RotTR on 360 was a good conversion too. Speaking of high performance profiles, RE7 is 1080/60fps on PS4, that one would be a prime example of a port that could be done relatively easily and would likely sell quite solidly on Switch and make a healthy profit.
RE 7 yes. Dark Souls 3 no. Dark Souls 3 requires a good CPU for all those physics and animations. That said maybe From is working on it and if Dark Souls Remastered is a success on the Switch they'll bring Dark Souls 2 SOTFS and Dark Souls 3.
Was the first year of the PS4 or Xbox One much better?
Yes the third party support was very good. No different than 2k17.
Doesn't look all that complex compared to the more high end titles from Western devs. Rise of the Tomb Raider looks more complex in most ways.
You could port just about any PS4/Xbone game to the PS3/360 if you were willing make enough cuts; remove and simplify effects, reduce asset quality and density, trim rendering resolution, etc. And Switch is a fair bit more capable than PS3/360. Doing a RorTR type down-port of something like Witcher 3 might not be worth it, but plenty of other titles could justify and recoup their porting costs and turn a nice profit. Whether third parties are smart enough to seize these opportunities is another matter.
Switch has a weaker CPU than PS3 and in portable mode I've heard that its GPU is less powerful than a PS3. It has more ram and a more modern architecture but I don't think that would be enough.
Look at Doom say. It runs at 1080p60fps on PS4 at medium to high settings. It runs at 640p30fps on Switch docked with lower than low settings. That with a very scalable and good engine like Idtech6. Unreal engine 4 games struggle to even run at 30fps at 1080p on PS4, it would get real bad on Switch. There is only so much downgrading possible before the game loses its soul. Those Wii ports of PS3 games felt nothing like the PS3 version. I don't think that is an ideal experience for a Switch owner whose spending $60 on a game.