I don't think they will disband as soon as Luffy get the One Piece especially since Luffy should get it before he even turns 20 at the current pace, he is not just gonna stop being a pirate that early. They will probably keep on being a crew for around 15/20 years or something, live a lot of other adventures. Then eventually they will disband to all live their respective lives. Franky will go back to his bros in water 7, Sanji will go take over Baratie and marry Pudding, Zoro might just move to Wano and marry Hiyori, etc...
Or they might stay together even longer, Whitebeard was a pirate until he died after all and it seems like Roger only disbanded his crew because he was sick. Luffy may never want to do anything else than being a pirate.