High Hopes, it's a bit more complicated than that......
What happens if the traffic on final decides to go around, straight into the path of the departing traffic.
The New Galloway departure from Glasgow goes straight ahead, the missed approach goes straight ahead and conflicts with the path of the New Galloway departure.
Options to turn the missed approach right (off 23) or left (off 05), however this then puts traffic toward the hills. Left off 23 or right of 05 put traffic into conflict with inbounds being vectored for the ILS.
Our job is about safety, far too many incidents have happened because a controller got so committed to a course of action that was never going to work and didn't recognise that fact. The safest option if time is available is for the traffic on the runway to vacated at 'B' (on 23) or 'F' on 05 or as has been stated to stop the departure and send the inbound around.
In my opinion the controller made the decision based upon the facts presented top them and decided on the SAFEST course of action