You made some points but I think the 3DS isn't the WORST enemy. Because with your logic in generation 7 the DS and Wii should have hurt each other, too. But both systems were the best.
So that means we should look for some other reasons. Both Wii U and 3DS sold bad at the beginning but the 3DS managed it to find a lot of fans. For comparison we can say that the 3DS is a better DS and the Wii U is a better Wii. The better DS sells better than the better Wii. And why? Because of the oppurtunities. Vita or 3DS? 3DS because of better games! But the Wii U has not only one weak enemy: it has two of them with the PS4 and the Xbox One and this time the Casual gamers won't help because they are saticsfied.
Of course there are other reasons like the bad advertisement or the misunderstood Gamepad. And I also think you are right when you say that the 3DS is also an enemy but I don't think it is the worst.
I don't hate Microsoft, I don't hate PC,
I don't prefer Sony, I don't prefer Nintendo.
...Ok, I love Nintendo but this is something about tolerance, ok?
I'm a gamer with one of the greatest hobbies and I want to share this greatness with everyone.
