It's hard to define "successful", it is just simply that OP as one of the potential mangas was "luckily" picked by Jump to be their mascot, invest money and milk it later. The quality only mattered at the beginning and after that it's all just marketing when it got a number of diehard fans, the manga only need to drag on as long as possible to make more money. So OP at this point is not a "successful manga" anymore, it's more like a undying slave or a money addict, not leave resources for other mangas to grow.
It's not about luck for them, they were just the best stories available at the time which meant more people were buying volumes and merch. So I wouldn't agree with your idea about luck.
However, I do agree that it has dragged on longer than it should have in retrospect. But why leave money on the table when you have a cash cow ready to be milked? Even once the manga ends One Piece will stay a lucrative money making scheme with dozens of possible movies, spin offs, exclusive events, special volumes, and, let's face it, a remake of the anime down the line.