I agree with you that they need to sell as many units as possible, so that potentially influenced their decision to release it on both platforms. (It's not like any significant marketing budget is going in to pushing the Wii U version at this point.) There is also probably a lot of data to support that hardcore consumers may double-dip. They have been promising Wii U fans this game for ages, and cancelling it would certainly disgruntle an already jaded hardcore fanbase, so that may not have been an option. Releasing exclusive content on the Switch version (which, they still may do) would also agitate Wii U owners, as they may feel frustrated that they were receiving an 'incomplete game'. The only way to create a disparity between the two versions without directly drawing the ire of their fanbase would be to blame the gameplay/audio/visual performance on the hardware -- whether this is true or not.
With all of that being said, think of the optics of releasing the same game on your own two consoles, released in 2012 and 2017 respectively, that run virtually identically. It is absolutely in their best interest to ensure that the version of the game that runs on their new hardware have better performance, even if this is artificially implemented by capping the output of the other version. For better or worse, this Zelda is about moving Switch consoles at this point.