Surely the reason the 'slot' system was initiated was to eliminate the chaos of too many departures/arrivals at 'optimum' times; the limits of capacity rules. If, for whatever reason, a carrier doesn't use a slot, that's capacity unused, which defeats the system. Surely, only political pull (or something similar) would allow limited capacity to go unused for long; if 1 carrier can't profitibly utilize a slot for an extended time, another must be allowed in.
Otherwise the industry would stagnate.