Bad news for the travelling public. A fantastic airline disappears, its staff end up jobless, BA has less competition and becomes more arrogant and highhanded in its dealings with its 'customers', its rivals, and the travel industry. Fares go up, choice goes down, and a part of aviation history disappears. And BA control 56% of LHR slots - surely that can't be healthy.
BMI are not fantastic and have not been so since the rebrand where "BMI" was not an acronym and "didn't stand for anything". A truer word was seldom spoken....
BA and BMI do not compete on more than a small handful of routes, they are complementary not competitive.
Most BMI crew will be needed to fly the program to keep the slots.
Fares were kept high in LHR-GLA when the demand vanished but right sized when BA added in the B767 to up capacity. easyJet are a tough competitor in that market and make sure there is no monopoly, indeed far from it.
Any monopoly gained today will
ONLY be in routes ending in "-stan"!
As has been pointed out, controlling 56% of slots at your home base is less than say :
LH at FRA
AF at CDG
KL at AMS
AA at DFW
UA at EWR / IAH
EK at DXB
QR at DOH
EY at AUH
FR at STN
Aside from all of that your analysis nailed it.