Then it would continue the Nintendo console sales decline that the Wii broke from being a casual magnet. Then there would be no 8th gen home console from Nintendo.

No.

Please elaborate.

The losses they are taking now is chump change compared to what they made.

The Wii U is a successor to both, and is a nice fit. When I think of the Gamecube, it was a hardcore console. Resident Evil Remake, Resident Evil 0, Resident Evil 4, Star Wars: Rogue Squadron 2, Eternal Darkness ... When I think of the Wii, it was more softer. Punch Out, Super Mario Galaxy, Super Mario Galaxy 2, Wii Sports, Sin and Punishment 2 ... The Wii U has the best of both. Super Mario 3D Worlds, Donkey Kong: Tropical Freeze, ZombiU, Deus Ex: HR Directors Cut, Lego City Undercover ... Many developers have already said the Wii U is powerful. Retro Studios called it a powerhouse.

Well Nintendo DS would've been strong as heck and maybe they could've pulled it off better. Wii U would've been the strongest since it had the most RAM and a lot of other stuff so I'm sure 3rd parties would've attracted to it. All they would've needed was a good game and it would've sold like crazy

You mean another console made specifically for children like the GameCube? No thanks, I'll take the Wii U any day over some other GameCube.

Not if you factor future losses. Nintendo may have cut their own hamstring with the Wii by severing so many ties with consumers and developers. A Wii U successor to the GC would probably sell A LOT more software than the Wii (which honestly didn't sell much, a lot of casuals bought it just for Wii Sports), which would mean that Nintendo would make a similar amount of profit assuming they sold around 70 million Wii U units. Not to mention the fact that they could be on foot with Sony, if not doing better than them this generation. 

If the Wii U came out earlier, it would easily beat the Xbox 360 in the first year (Motion controls alone would have probably pushed it ahead) and would get a huge start on the PS3 since it was so expensive starting out. Nintendo may still have won last generation, even without the casual support. It would also have a lot more momentum going into this generation. 

Did you even read the OP? It would be a console meant to appeal to third party devs, making it more geared towards core gamers.

Also, since when was the GC made specifically for children -_-

You must have been a PS2 owner and overlooked the Gamecube.  That was a really hardcore console.

It would have failed compared to the Wii. The gamepad/tablet is not nearly as compelling to casuals as motion was. I mean especially with touch and two screens already available on their DS, I'd suppose. And at that $400 price... I mean, right now the WiiU is just a DS for your TV most of the time. The gamepad has not been used fully for its mobility/asymmetric play in many games, just HUD/map/menu/drawing.

Also, Nintendo released a console notably more powerful than the PS2 with the GCN, and it still didn't get massive third party support (it still got darn good third party games, but it's no PS2). Getting third party support from publishers would not magically happen if the Wii was the WiiU instead. Nintendo would have to work on business relations, not console specs.

Plus they would have lost money on every console, in significant $50+ amounts. And if they maintained a similar online architecture as they have today for free that would have run them a pretty penny 7 years ago as well. Probably one of the few cost mitigators 360 had for MS early on was pricey online.
Nintendo didn't have a super strong market to build from off GCN. Xbox managed to do it, but look at how much money it took them. And they got many third party games on 360.