surely if IAG buys BMI they cant just buy the slots... they would have to take the a/c and crew with it. And there are laws pretecting the employees so they cant just be made redundant.
I think the two closest analogies would be :
1) Dan Air which BA bought for a £1, kept some routes, crew and B737-400s and that was pretty much it and...
(this one's MUCH worse !)
2) Buzz which Ryanair bought off a European flag carrier, KLM and took on some routes and crew and let the rest go, including all the aircraft.
Classic F.I.F.O. from Michael O'Leary to the staff who threatened to strike.
However in the case of BMI, the A320 series aircraft would be a good fit into BA, indeed some were delivered to BMED in the current BA scheme.