SOU and BOH are each owned by competing private companies. Neither will want to hand their business over to a rival for free. Govt is not about to pay £100m+ to compensate for intended change of use. There are simply far too many entrenched interests who would demand far too much money for SOU to be closed, never mind the monopoly issue
If an airport can pull in over 1m pax per year, there are likely good reasons for the airport remaining in place, regardless of issues that may exist.