Don't worry, they for sure have a replacement for one piece. Those big money guys at the top are not stupid, the have backups on backups ready to deploy. You can't even imagine how many young talented writers in a country with population of 126 million exist. All those writers keep sending their stories to publishers in an hope that they will accept it. The big money guys take those stories and keep'em in the backup rooms, in case things go wrong. One piece is already losing its popularity, slowly but surely one piece will fail to deliver considering Oda's stupid writing in past 10+ years. The quality of Oda's writing has gone down significantly since timskip (that's why Oda has been pushing fanservice so hard, cuz it sales). Naked Robin and Nami, shirtless Zoro and Sanji with musicals etc. Same thing can be said about confusing power scaling that is purely made to keep the audience busy arguing against each other so that they wouldn't pay attention to the poor writing quality. New characters like, Carrot, Yamato, Ulti, Black Maria etc. Are another way to keep this sinking ship floating little bit more, to squeeze more money, as long as they can.
And when the ship finally sinks they'll open the back up room and unveil new great story that will sale the fuck out of readers.