Indestructible implies “can’t be sacrificed” which various spells and abilities use to ignore units. Glaxx’s ability could do the same when gold is present, and then those cards and abilities could all just look for “can’t be sacrificed” and work as intended. Granting indestructible to Glaxx or giving him massive XP would make Glaxx survive damage he shouldn’t be able to and it wouldn’t prevent other removal effects like trashing and returning to hand.