United pulled out for a reason and I doubt it was an untested airline like Primera that scared them off.
I've recently been thinking the same. They were fine when American showed up so why would Primera be any different.
Interestingly I was looking at some numbers earlier and in 2016, between United and American, BHX saw just over 200 each day in each direction to NYC - certainly enough for a daily 767...
Jetblue next anyone?