When I touring colleges with my oldest son, he fell in love with a college for which we’d clearly be full pay with little chance of merit. I explained that if he were to go to this college, he’d be using up not only all the savings we had for his college but also the college savings of his little brother as well.
His compassionate response was essentially “You always say [little brother] will be fine no matter what he decides to do! Give me all the college money and he can be a plumber or something. He’ll be fine!”
Later on, little bro (who was 10 or 11 at the time) overheard this story and indignantly burst out with “but I don’t wanna be a plumber!”
So of course "but I don’t wanna be a plumber is a family phrase for when someone is being a wee bit selfish…