Why do people go on and on about SOU-MUC. The airline tried it for goodness sake. Frankfurt and Zurich were also attempted 15 years ago. If the yields were 'improving nicely' and 'it was popular' it would still be going. But it isn't because it wasn't viable. So either yields were not improving nicely or the passengers continued to use LHR due to the flight frequency choice. Either way it was not viable. With LHR just 60mins up the road SOU is going to struggle big time to go beyond 2m pax especially when considering the 3rd runway looming.
Therefore there is a finite level of demand at SOU - as other posters on here have mentioned. SOU is the ONLY airport in the country that is 1hr drive from LHR but not classified as a 'London' alternative airport. It's suffering from the 'black hole' effect that is LHR and LGW. Just look at U2 on SOU-GVA. SOU-GVA can only support 3 times per week, yet BOH can support 5-6 times per week as it is more distant from LHR and LGW & attracts a more western geographic catchment area. People in London are very unlikely to consider traveling to SOU to catch a flight.
I'd also be alarmed that the CEO up and left seeing more potential at BRS than SOU.