Let me show you exactly why Oda hasn't planned this properly
here they are very dismissive of the fruit saying it's a legend that hasn't awakened for 100s of years EVEN FOR THEM.
then in the spoilers we have this
"The “World Government” tries to keep the “Gomu Gomu no Mi” with them in every era... But, somehow, the fruit always gets away. And 800 years have passed...!!"
Now the same fruit that they were soooo dismissive of they now talk about how it's so prized and how they try to capture it in every era.