BA's control of LHR slots is still not as good as many other airlines have at their hubs
Market dominance is rarely a good thing for the consumer.
Also, it is quite possible that IAG will use BMI as a means to lower costs within BA and take over the BA Sh network making them effectively a LH carrier only.
That would be a good outcome, I think it unlikely.
BMI are being bought by IAG, not by BA, that's true, but who's the David and who's the Goliath here? My prediction is that BMI will be swallowed up by BA. Watch this space - I hope I'm wrong.