When I say that GC and N64 were failures, I talk about sells.
I had a N64, and my favorite game of all time is Super Mario 64, but it's undeniable that PS1 outsold N64.
The same way PS2 outsold GC.
If third party developers don't support WiiU, it may come to a failure in term of sells, and this is why Nintendo is trying to have third party support.
Both were not successful because of the non-support of third party developers. Third party developers didn't support because of the limited space of the media in both.
Wich means that your sentence ("the thing that held them back was the media format they decided to use") is right in a indirect way.
The most part of high quality third party games weren't supported by Wii because of it's weaker hardware, that's all I'm saying.
I don't see why you question such facts