Usopp is everything but a coward. He is scared, a LOT. But NEVER a coward. Even when his body tries to run, he comes back around.
Usopp will never become stoic, that's not his character, but he will learn to be proud of himself. And for that.. there needs to be a bit of a development and it will most likely involve his father.
Remember, we do not "grow up" by becoming stoic, we simply shut down our fears. Stoic people are not necessarily the courageous ones. Usopp is always afraid, but he NEVER lacks the courage to do what is necessary.
Because Sanji needed to understand his own worth.
Sanji is all about auto-destruction for the sake of other, giving up one self for others and letting the words of others define him. Sanji had to be reminded that he is part of the family now, that he can't give up his life, that he was worthy of living and having a dream.
Hence why to wake Sanji up, Luffy implanted the thought that without him, he would die.