Well the think the honest truth is that a company can keep a series going FOREVER if it keeps making them money. Without a doubt some series, like One Piece, are set up in a way that they can last longer. But, yah, once you use up every idea in existence... what else can you do without changing the series' setting/characters/etc...? Look at the Simpsons for example: It's on it's 24th season now... and it's not nearly as funny as in earlier days. It's cause we've known the characters for so long that we really know everything about them. There's no real surprise. Have you heard of the rule of three? It more or less says that you can repeat a joke about three times before it starts to get old. Think about Brook's skull jokes for example. The reason I think that One Piece still holds our attentions is that it has an amazing plot and very memorable and powerful moments that keep us wanting more.
Even for myself, just having Luffy burn down a town or enslave a civilization will eventually get old. I keep trying to come up with new ways of making him do evil things. I think all series run out of ideas eventually.