It becomes a fact when the Wii U has done so. At its current trajectory, the Wii U will fall short.

Let the disappointment ensue.

I'd say the Wii U has no chance of getting past the Gamecube. Thats still over 12 million sales away. Even if the Wii U from here on in was to average 50,000 a week worldwide (which in itself is VERY optimistic), that still means its got to do so, for 240 more weeks. Thats well over 4 and a half years! The Wii U will not be still on the shelves in 4.5 years time, that much is pretty much certain. The Wii U is destined to become the worst selling Nintendo console of all time.

Wii U will fall short of this goal

It's great to be optimistic..

...but pessimistic people like myself does not even think the Wii U will reach 15M

They don't really have much choice the way I see it, it's either a Fusion concept or streaming with different dedicated devices. They have to carry their platforms virtually on their own when it comes to software and with the extended development cycles since the Wii days and more than one platform to develop for; they have to find a solution to their current game output being too slow.

Out of the two options, streaming vs fusion; I believe fusion is more likely, for someone with as little pedigree and as few allies in the world of online infrastructure as Nintendo, fusion would be easiest and safest bet. Streaming would be too radical.

It's really sad when a Vita fans such as myself feels bad about the sales and fate of another console. I was sure that Wii U would overtake Vita at some point due to the push of outstanding software. In fact I will buy a Wii U when they set the price at a reasonable level, but after the NX and mobile announcement I think Wii U will be axed much earlier than we expect. It's probably better for Nintendo, but worse for us. Makes me wonder how many games it'll get in 2016, let alone 2017.
To sum up - at this point I don't believe getting near to GCN is possible. Wii U will fall short and Vita will remain ahead due to its much longer shelf life, lack of a Sony next-gen handheld and quite powerful hardware allowing it to get all indies for like forever.

IMO Nintendo has abandoned all hope for Wii U and they will only support it over the next 2 years to the extent necessary to allow their next hardware move to not launch in a complete Nintendo vaccuum. Which IMO means trying to sell about 3 million consoles in 2015 and in 2016 after they announce all new hardware to sell another 3 million Wii U's between the announcement and launch, which will be about 18 months. So 17-18 million Wii U's sold LTD by the time their next gen starts, and a rapid exit from the market after that.

Apparently the WiiU is showing signs of hope. So... maybe? If it find it's Unity (!?) groove, might make it to 64 territories.

Haha it will top out around 16m.