'320's (and '319's) routinely fly TCON's on DL and other carriers and that's the way things are and will remain. The days of flying excess capacity are long gone.
Once in a great while, this will result in weight restrictions before loading and that's a bit of a hassle. Enroute, maybe a fuel diversion and a bit of a greater hassle.
But, that's better than losing a frequency which is the alternative.