As for Epic Yarn being a new IP: What would be the point? Prince Fluff already looks like a blue Kirby anyway, so why make a new character who is so similar to an existing character? Fluff is still in the game, and I have my doubts anyone would have paid attention to it without Kirby.
If Nintendo tried they could probably shoehorn an existing franchise into any new game they made. But if they did that back in the day, there would be no Kirby. At some point they had to take a risk and go with a new IP. All those established Nintendo franchises were brand new at some point and Nintendo will never create the next successful new Nintendo franchise if they just use the existing ones all the time.
Personally I'm just burned out on the same Nintendo characters. I remember when almost all of them were brand new (Mario and DK are the only ones that just "always" existed for me) and that used to be part of the appeal of Nintendo. They had these iconic franchises like Mario and they were also introducing new ones with new heroes, new gameplay, new stories, new villains, new worlds. Since Iwata took over that has pretty much disappeared (though on the N64, Rare was largely responsible for the new IP). In retrospect shoehorning Star Fox into Dinosaur Planet was pretty much the beginning of Nintendo's "franchise factory" approach and the Gamecube's "who are you?" marketing campaign, that played exactly to Nintendo's negative image of making cookie cutter sequels, soon followed.
It does depend on where this Kirby was at when Epic Yarn was made. I understand that if this was vaporware, there would be a desire for Nintendo to have some sort of Kirby game for the Wii and this would work. But if this was close to being ready, it comes across as being gun shy about new IP and using the existing franchises as a crutch. Creatively Nintendo will stagnate (hell I feel they have been for years) if they're afraid to break away from the established franchises.
Nintendo of today would probably have never made Kirby in the first place because the traditional Kirby gameplay of inhaling and exhaling items and absorbing powers could easily have been fashioned into a Mario spin-off staring Yoshi. Star Fox could easily have been a Metroid spinoff with Samus flying around in her ship.