Whitebeard
could have awakened his devil fruit, but he is not the type of man who wants to destroy the world. Oda is saving
that kind of stuff for a person like Blackbeard, who is very interested in plunging the world into darkness. Hopefully, we can see its true strength.
Did WB spend decades becoming very proficient/mastering the fruits power? I would think so. He's able to touch and hold air, imbue quake bubbles on his bisento, and control the magnitude of the vibrations to name a few techniques.
At the end of the day, we have been shown that WB doesn't need to awaken the devil fruit in order to use it the way he wants. Blackbeard, on the other hand, has to
RELY on such a fruit to succeed in his grand ambitions. Let's wait until the story progresses a bit.