It's a little sad to see things the way they are. Nintendo has a load of beloved IP, and a generation of developers who would likely love to build for a Nintendo platform. But they've failed to capitalize on either.
You have to realize their history though. The "Nintendo seal of approval" exists because the game market was filled with horrible games when they first started, so trusting some other dev with their IP, which could potentially be ruined by a third party. They have been making strides with this (see Hyrule Warriors), but considering the piles of garbage games on the app store, their aversion is understandable. On top of that if you could get Nintendo games on other platforms, their console sales would suffer. Plus, a touch screen would just be frustrating for platformers like Mario, only turn based RPGs seem to port well.
I'm not suggesting they trust another dev with their IP. Perhaps I phrased things poorly. I think it could be potentially worthwhile to make their platform more open to allow devs to introduce their own IP on it. I also think that Nintendo could be doing more to get older gamers who grew up on their games to play Nintendo again (including, yes, potentially making games available on non-Nintendo platform).
Would console sales suffer? I'm not so sure. The market for, say, tablet games may not be the same market that exists for Nintendo DS games or WiiU games.
More than anything, it seems that Nintendo has just decided to do nothing to fix the situation. Even small-scale experiments in expanding their marketplace reach or opening up their platform may potentially encourage investors in their ability to get things back on track again.