It's probably safe to assume that Zelda, as a video game series, won't end as long as Nintendo is still in business. Will it always be on Nintendo consoles? Honestly, I doubt Nintendo can stay in the hardware business for very long; sooner or later something is going to give, and Nintendo just won't have the resources that other media companies have to keep themselves floating in the console business. When that will happen is anyone's guess I suppose, with some saying it won't happen for a long time, and others saying that this generation and the WiiU will spell the end of it. I personally have loved every piece of Nintendo hardware I've ever owned, but that's just me.
If or when Nintendo does quit the console business though, Zelda will definitely still be around, hopefully with the likes of Mario and Metroid, but just on another game system. I would question the assumption that the quality of the games would continue at such a high level (took Sonic a long time to get back on track once Sega stopped).... But they'd still be in production unless Nintendo just calls it quits one day. Zelda, just as a name, still carries enough weight to give it some pretty good longevity (Metroid not so much, unfortunately).