And do you know why I insult (Set FAIL For) One Piece, and adore Ah My Goddess? Because AMG has everything OP doesn't: attractive characters, romance, perfect art, action, fanservice, hot girls & more. And above all - it's reverse-shonen, insofar that the hero gets his wish from the START. If only OP, Naruturd, Blech, & all other Shonen crap did that, then maybe they'd be tolerable, and there'd be no need to plough through interminable filler, 99.99% of which has little or no relevance to the plot. You wouldn't have to keep buying more vols because you know the hero has what he wants. At least you have that choice. But no. You don't know WHEN Naruto will become the Fouth Hokage. You don't know when (indeed, IF) Luffy WILL find the One Piece & become King of Pirates, and so you have to keep emptying your wallet, in the vain hope said story MIGHT finish.
And there's something AMG has in abundance that Shonen will NEVER have: true cross-generational & demographic appeal.