Pretty much every crew member has a reason to stay on the boat even if/when they achieve their dreams if you think about it. And I mean aside from because they want to help their friends to achieve their dreams and because they want to see the world and what not.
Luffy: Wants to continue having adverntures, defend his title as Pirate King, meet new friends.
Zoro: Wants to defend his title and continue to grow in strength and finesse as a swordsman. And with the world as it currently is, being a pirate is the best way to do this, and if he's going to be a pirate, he may as well be one with friends.
Nami: More adventures means more money, and more chance to refine her map(s).
Usopp: When he manages to prove to himself that he's brave and strong, would he want to go back to Kaya, or to his father, or would he want to use that strength and bravery to help his friends?
I know which I think is more likely.
Sanji: Like I said, he'd want to continue adventuring so he can learn more recipes and styles of cooking, as he did in Skypiea so as to become a better chef.
Chopper: Search the world with his friends to see if there any diseases he personally has never heard of to find a cure for.
Robin: She said it herself, the crew are basically her only real purpose in life at this point, and while she does have a dream now because of them, she has shown that it's less important than her crewmates through her actions at Water 7.
Franky: IF he were to join, and subsequently achieve his dream, it'd mean he's already reached Raftel at least with them. After that, he could stay with them to continue improving the ship maybe. But he at least has a good reason to leave them, what with the Franky Family, so he's the only one I'm really unsure of.
I've always thought that argument that such and such can't happen because if it did [insert name here] would achieve their dream. And if that happens they would of course immediately walk off the ship and abandon their friends was slightly retarded for the above reasons and more to be honest.