I would think DUB produces pretty good A321 loads as well, but that said, EI is also said to be giving up on A321s as well.
If BMI were to pull off the Belfast route (either BFS or BHD), it would be quite a surprise; if they are filling 321s, then 319s or 320s should not be a problem - and at better yields. Presumably the reason the 321s are disappointing is that they have to sell a significant number of seats at bargain prices to achieve satisfactory loads.
As for ending Belfast-LHR altogether, the political reaction to this in NI would be vocal and HMG would be under intense pressure to find someone to keep it going. One possibility might be EI, although I think given the choice of flying EI to LHR from BFS, one good reverend might rather swimming and cycling
