Ok, I understand.
I think your question is really just the same as the question us engineers have asked ourselves for a long time, to what degree does it make sense to make our mixes sound the best they can considering that users pretty much always have an inferior playback system. It’s true even for stereo where they listen on anything from earbuds to crappy phone speakers. At the heart of it it’s the same question applied to a wider format.
To me the more interesting question is how much resources (time and money) it takes to mix for 7.1.4 rather than 7.1.2. If it’s not much extra cost then I would think the answer to the question is that it’s worth it.
Or to ask the converse question: Why not mix for 7.1.4 so those with that setup or higher can hear the best mix possible?