The setting, pirates, imo is secondary in the end. What is important is Oda unique take on the genre, these are goofy pirates with devil fruit powers who have to travel from island to island etc. If somebody tried a generic pirate format you would end, best scenario, with something like pirates of the Carribean which is completely different from OP pirate setting. So there could even be such a manga and imo people would consider it and OP as 2 things completely different.