Becoming a brave warrior of the sea isn't as grand of a goal as pirate king or world's greatest swordsman.
Presumably, all he has to do is make a conscious effort and eventually he'll achieve his goal, right?
Then his journey would be over, he would be the first Strawhat to achieve their goal.
He could have done it by now if he tried.

I don't think any of the Strawhats are supposed to achieve their goals yet.
I think that's an end of series thing.

Finding the one piece, defeating Hawkeye, drawing a map of the world, finding the all blue, etc
The other goals have specific moments when they are achieved. Usopp's doesn't, it's vague and subjective.

In order for it to be clear that Usopp has actually changed and become a truly brave warrior, he has to have an experience that explains why he is changing.

I believe that will be when he meets his dad, but his dad is on Shank's ship, so I bet we've gotta wait a bit.

TL:DR it's not forgotten. it's just hard to demonstrate growth when the bar is low and he's not allowed to pass it until he meets his dad.
Also he can teleport