If BA were to withdraw from Dublin, it would be because they can't make any money flying there. Aer Lingus would therefore have a monopoly because of the workings of the free market rather than because of the ba/BMI take-over.
The EU can compel BA/IAG to make slots available to potential competitors on various routes, but it cannot compel competitors to lose money flying those services in competition to BA. We cannot lose sight of the fact that bmi has been losing money at an alarming rate and that, ultimately, is the reason why the take-over situation has developed to begin with! Why should others be able to make money on routes where bmi has presumably been losing heaps of it?
It is not unreasonable for the EU to ensure that people can compete if they want to, but the actual number of new airlines who will put that to the test - given that the LHR slots will be tied to the routes on which competition is deemed to be needed - will be remote, I think.