Ok 4 years not 5. Just because it was originally a Wii U game doesn't mean it shouldn't run and look significantly better if Switch is really 4-5x more powerful even with a year of porting (which isn't a rush job by any means).
BotW would be 1080p when docked with better shadows, better texture filtering, better AA and a solid 30fps were Switch as powerful as you say. Just because they're getting 1080p on Fast Racing and Mario Kart doesn't mean they will hit that improvement in every game because as you know different genres require different hardware needs.
If Zelda was rushed for launch where is the framerate optimisation patch almost a month later? I don't think it's a matter of a rushed port at all but rather Switch being a modest hardware leap over Wii U which struggles to run anything other than a closed circuit racing game at 1080p.
As I said earlier Splatoon should run at 1080p in docked mode. If it doesn't it will add further fuel to the fire that Switch isn't much more powerful than Wii U.
We know the Switch is definitely not powerful enough to bruteforce BOTW at 1080p, locked 30 FPS, and so forth. That doesn't mean there weren't ways to get there; it just means they didn't do them for reasons we don't know. There could easily be a bottleneck in their engine that they just didn't have the engineering time to deal with. That's the reality of software development. And if it's Good Enough©, they might not have any reason to go back in and fix it.
I think it'll be more telling to see if they make any improvements when the expansion pack comes out later this year. That'll require a client patch and we may see some additional improvements.