The piano comes with an heirloom and no you don't have to retire your sims for you get the piano heirloom. the piano heirloom unlocks the piano hobby. It's better to have your sim have a musical trait.
Your sims leave their own heirloom when your retire them. It may be an heirloom not associated with the hobby they were in. It depends what you have already unlocked in the game. I had a fashion sim that left me a trait heirloom that was the competitive trait
