WRT BMI
1) Don't forget who's trying to buy BMI - it's IAG, and not BA. Subtle difference.
2) IAG want slots - hence the purchase. They aren't buying BMI to close it down, their buying it to operate with. The slots HAVE to be flown on, else they are lost. So all of the BMI pilots will be required (to operate the current, or equivalent, BMI slots).
3) BA needed pilots to join over the next 5 years, there is no reason to suggest they still don't need those pilots - IAG/BA want expansion not retention. Hence the purchase.
4) Expansion is likely to involve more long haul routes which are more crew intensive.
I could be wrong - the above is just my humble opinion!