This is a senseless question for many reasons. First of all, we all know that Zelda will continue its stride. It is one of the longest-running game franchises ever, and I doubt there's much that will stand in its way. Even new releases like Wind Waker and Twilight Princess have received amazing critic scores, meaning Zelda hasn't lost its touch.
As for Zelda sticking to its original formula... I would say "no", frankly. Also, the formula has already changed many times in the past, going from 2D to 3D to 2.5D. I don't really see why Zelda should cling to its roots when there's really not much to dig up there. I say that Zelda should move forward, but still pay enough homage to the classics to please fans. We've all been witnesses to easter eggs in Zelda games repeatedly throughout the Zelda franchise's history. There are references and very similar gameplay mechanics, but I would say that Nintendo shouldn't go so far as to making Zelda games play like God of War... *shudders*. But I do feel that Zelda games should occassionally stray from the ordinary, just to keep things fresh. We all complain about defining yet underappreciated games like Wind Waker and Four Swords Adventures... but the truth is the uniqueness and change-up in graphics, gameplay, etc. really keep the Zelda franchise from dying out. And with Skyward Sword on the horizon, I doubt the long-running Zelda series will come to an end.
As for outliving Mario and Metroid... most definitely. Star Fox? Star Fox is a franchise that is nearly extinct and will most likely remain so. I have no clue why Star Fox was even mentioned, since it's not that long-running and only has a few installments. Anyway, Metroid is a single year older than Zelda, but also manages to stay at the top of Nintendo best-sellers (not really). Zelda will outlive Metroid... without a doubt. Bigger legacy, more fans, better games, better critic and fan scores... there should be no reason why Metroid outlives Zelda.
As for Mario... that just cracks me up. I feel as if Mario isn't going to die out anytime soon, but I feel that some gamers are growing tired of the Italian plumber's antics. There have been too many "trashy" games released as spinoffs and too many terrible games that sport the "Mario" name in general. The truth (which most don't see) is that Mario's time is nearly up, with only few quality games remaining in his arsenal. Mario definitely was better in the old days, not overused and overly recognized as gaming's biggest face. I see Zelda outlasting Mario solely because Mario has starred (or co-starred) in over 200 games, and his flare seems to be dying a little in the hearts of hardcore gamers.
