There's a difference.
Card Captor Sakura was fail and so the show got canceled.
One Piece really is fucking great as it's about the creative stories about the crew and their life.
Why would you stop an anime if you can do so much with it?
In One Piece you could do so incredibly many different things, it's awesome!
This show could run endless and wouldn't get boring as this series actually IS about seeing as much different things and live through as many adventures as possible!
I enjoy One Piece very much as Oda's ideas are always fresh and new, the arcs are all different and the characters stay, of course, the same.
Why would you cancel it?