In regards to the touchscreen controls, that is more of a play style/feature than an entire game mode. You are right that some games do offer that, I mean Okami HD, which was in the Smash Nintendo Direct, offered touch controls in handheld and motion controls when docked. Thing is though that this does not lock the stage builder to handheld or docked, it just means that touch can be used when in handheld.
I would like to clarify that I do think it is possible for them to get 8 player Smash working in portable mode, I just do not think that it would be an effective or enjoyable experience and as a result may be cut. QoL type of thing so it does not make the game look bad.