Perhaps a little historical information would put the present in context.
In the seventies, when BOAC and BEA were amalgamated into BA, two other companies, Northeast and Cambrian, were also included. The seniority lists were integrated on the basis of Date of Joining.
Following the amalgamation, routes were dropped and fleets reduced. BOAC and BEA first officers lost out on time to command. Some of the Cambrian and Northeast captains did not have the seniority to retain a command in the new and much larger BA but 'redress training' back to the RHS took time and was received with bad grace by one or two who should have been grateful that they were now BA employees.
ISTR British Caledonian pilots were treated in a similarly agreeable manner (for them) when B-Cal was taken over by BA.
When Dan Air was taken over by BA the story was different. Those who were not qualified on the appropriate aircraft were not offered employment. Some Dan Air pilots had voluntarily forgone the conversion to which they were entitled and were, therefore not offered employment; a signal lesson to me, at the time, in looking after one's own career!
I cannot remember if those ex DANs fortunate enough to be employed were integrated into the seniority list or joined at the bottom; I seem to recollect the former.
Which brings us to BMI.
AFAIK, the BMI pilots became BA pilots but joined at the bottom of the seniority list. The BMI captains retained their commands. If I had been one of them, I'd have been happy with that.
A lot of one's whole career depends partly on competence and largely on luck and choices made without the benefit of a crystal ball. I've had colleagues join British companies which went bust with no BA safety net and others who joined, e.g., Emirates, right at their inception when the rest of us were saying "Ooh, err, new startup - risky." You pays yer money and takes yer chance.