The GameCube was most definitely not positioned as a product for the core audience. Anyone who wasn't too young at the time should remember this very well. The GC was positioned, first and foremost, as a console for kids and families. That was Nintendo's message (and the main reason why the console became a victim of horrible market perception), during a time where the videogame audience, including Nintendo's own fanbase, was getting older than ever.
Nintendo's core games where getting outsourced at the time because EAD was too busy with shifting their creative efforts towards casual games, like Animal Crossing and Pikmin, gimmicks, like Bongos, turning their biggest core series into a game with toddlers, and underplaying the importance of graphics (certainly not something that the core audience wants to hear). Nintendo did fund a few core games for the GameCube, but this doesn't change anything (they did that for the Wii too). Having some core games on your console is one thing, communicating the message, building your ecosystem and positioning your product around them is another. The GameCube was most definitely not positioned as a go-to platform for core games.
Wii U was a product designed to cater to the Wii audience with core gamers being a far afterthought. The specs, the super basic online infrastructure, the tablet controller, the aesthetics of the OS, the marketing, even the look of its game cases, all scream what the target audience is.
Hell, almost 2 years after its release, there hasn't been a single 1st party game that isn't targeted towards casuals, kids and families. Nintendo themselves have admitted so.
No I'm not "kidding you".
As much as you don't like to hear it, the N64 didn't fail because it wasn't marketed as a toy for little kids and families. It failed because of NCL's incompetence, arrogance and a series of ridiculous mistakes they made at the time. It also performed very well in the west (much better than the family focused GC and Wii U) PRECISELY because of big releases that didn't only target little kids and families. It was in Japan that it bombed hard because NCL was a mess. They lost 3rd parties and Japanese gamers were too busy with playing tons of awesome games from Squaresoft, Capcom, Konami, Namco and Enix on non-Nintendo consoles.