So you're assuming that, because Zelda is a successful series, SNK screwed up by not making more of them?
That's kind of silly. Firstly, there are many series similar to Zelda, and there have been even more games similar to it. Not all of them have been successful, and fewer still have survived until today. Secondly, the biggest problem SNK ever faced financially was the transition to 3D games. Just because Zelda was able to manage this transition successfully doesn't mean Crystalis would have. There are countless examples of other successful (2D) franchises that received disastrous 3D versions.
You also seem to be forgetting that, before success of the DS and Wii, Nintendo's future wasn't always so "secure." Even Capcom, with its numerous blockbuster IPs (RE, SF, Mega Man) has a gloom surrounding it these days.
It's foolish to think there are any shortcuts to financial security.