I don't know how many you can cram into a one-class Euro lo-co 321, but US carriers keep the capacity in the 180-190 range. Once you hit 200, you have to add a 5th cabin crew, and it's not worth it to squeeze those last few seats in.
UA is taking on 739ERs to replace 752s in that capacity niche. NK is adding 320s (with a lot of seats); not 321s.