I agree with your OP mostly, the point I see it from is that if we are going to have romantic relationships, the dark sides of politics & corruption and other stuff that we usually don't see in utopia like children's worlds then it really can't hurt to have mild gore/profanity in the game since it will probably just strengthen some already mature aspects of the game. LoZ could go either way, dark or light but either way I very much doubt a dark game would change the core feel of Zelda which is after all to fight monsters in dungeons and save the princess from the bad guys. Personally I'd want something fresh which would preferably be a darker game, more serious characters that have more human feelings we can relate to.
tl;dr
The series doesn't need a darker game but could be just fine if not better with a darker and more mature game.