ex Trombone player here, Brass Band and Orchestra also played Tuba. It all comes in useful, the theory, playing as part of an ensemble, knowing how light a shade make a piece, learn how to practice properly, stage craft etc etc
Playing Trombone in a functions band at a high level was invaluable, sight reading on the fly, respecting other performers on the Bill etc stood me in great sted for running a band for thirty years on Bass .
I Think Piano or Guitar would have been more useful to go over to bass but the other lessons i learnt along the way were great.