investors are angry because Japaneses like to use their smartphone and not carry yet another device for gaemz...
and i agree with you and understand the investors part...
nintendo had an oppurtunity here, what about an actual tablet and smartphone (android or modified android) with detachable joy cons..
the smartphone could hide the controlls in the back of the device and you could pull them from the sides (which would change to game mode, a custom menu to open games)
like the psp GO, but instead of opening up and have the controls in the bottom, it has them in each sides!
Purely anecdotal, but in the 4 months I was in Japan, I saw a lot more adults carrying around a 3DS in public than I have in the US since the 3DS was released. While your argument generally makes sense, I don't think this is a situation where the Japanese market is the anomaly, and I highly doubt you have facts to back up that statement.
In terms of portability, a tablet is not as portable as a phone and hardly more portable than the Switch itself. There would have been no portability benefit for them to make it a true tablet device instead of a dedicated gaming platform. The trade-off would have been much worse, as being locked in to the Android ecosystem would have created numerous problems for Nintendo. At the very least, there is a good portion of the gamer demographic that buys console games
explicitly because they have their own dedicated platform, and are not part of some open round-robin of mixed quality like Android.