My points all originate out of the same two issues, SOU needs to invest and BE needs competition.
What do you want SOU to invest in? There is a motorway at one end of the runway and a railway yard at the other and constraints on runway aids that major customers couldn't use if they were there.
Which airlines are you thinking of that would do a better job? You ignore the fact that other airlines in the same position as BE (aircraft and crew out of place) do exactly the same thing.
The only argument seems to be that BOH spent £50m so SOU should - as my mum would say "if BOH put their hand in the fire, should SOU?"