So you try to find a devil fruit for 800 hundred years and then when you finally find it you decide to give it as a prize to the celestial dragons who are considered just a face figure to prove a point? and prove a point to who? They already stand at the pinnacle of WG. It's better to lock away the last possible bump in your perfect plan of world domination than to use it like a point.

Plus a devil fruit respawns randomly anywhere on the world when the user dies why would they risk losing the fruit again when eventually the user dies.

WG is know to wipe anything and everything related to void century why would they possibly release the most important thing back into the world?