I think the issue here is, which way round did things happen?
Did UA and TS (and later FI) inform BHX they were pulling out regardless, and thus BHX scrambled for any replacement service they could get
Or
Did BHX approach Primera to initially boost TATL, and then the other carriers got wind and pulled out on the back of it? It cannot be a coincidence that UA pulled out not long before the PY EWR announcement, and then TS around the same time YYZ was announced (and there was a decent time gap between the 2)? Let’s face it, airline negotiations do often take months, if not years, so there is just as much chance of either scenario being true.
If it is the latter, and PY do have issues, then BHX have potentially done something quite silly in hindsight (letting go of the birds in your hand to get more out of the bush).
But, If it was the former, then it was a risk that BHX had no choice to take and thus they cannot be lambasted for grabbing whoever offered a flight to them. Beggars cannot be choosers and all that.