Jackie seemed to run a tight ship until he was sick, but one has to remember he was sick for about a year on and off before he died. He was in and out of the hospital and it had been causing instability for a year or so. Tony was appointed Acting Boss while Jackie was first hospitalized (Pilot), but by the time Jackie was back on the streets it wasn't clear who he'd pick and it seems that once his cancer came back, it progressed so rapidly he didn't really have time to appoint a successor much less make other future plans.