You need new ideas to justify the new IP. New ideas are needed far more often than new IP. But you need new IP too.
Splatoon is a new idea that doesn't fit in one of Nintendo's established canons, and slapping a mascot on it would have been like slapping Star Fox on Dinosaur Planet (something that never should have happened). Similarly, Project Steam doesn't seem to fit any current universe, especially not one under Intelligent Systems' umbrella, so it's great to pursue the new IP. Captain Toad is an unexplored character that works great for his new game, which is sort of a "new IP" in many ways. Rhythm Heaven is a great newer IP (from the GBA) that I think would have been lessened by a familiar character.
You need new IP or else your old IP will get muddled and have no consistency as you try to slap a single name and character on every new idea.
Nintendo doesn't have an endless pool of worthwhile old IP. There are only a couple decent ones they own that they haven't revisited at some time in the last 10 years. And some of that established IP simply doesn't sell very well anyway (Custom Robo, Sin & Punishment).
So no, new ideas are not enough. But they are needed.
