Thats cool but has zero impact on discussion.
And this is where you are wrong as well. Smaller teams size is better for game for about 100 reasons with biggest on of them is social aspect. MMOs are not suppose to be competitive games where balance and shit is primary concern as it will never be fair unless you remove all loot and make every class just a template. It is suppose to be fun experience with friends. This is something that 10Man does a lot better.
So it is better for game.
M+ doesn't fill that niche as its simply different game mode. Its like saying you could substitute bread in your diet with grass. And here is your fallacy. There is not just 2 options. 3rd one exists as well, try to change it back how it was suppose to be, how originally blizzard planned it, when they still had vision.