It is fair to say that this is pretty normal practice, during the Airport Commission process (remember that big development debate!?) easyJet came out publicly against LGW development and BA against LHR, neither airline wanted the additional capacity that would give competitors free reign at their main bases.
In the end still nothing has been built, nor is likely in the foreseeable.
The good news for SOU is that for the time being, the existing infrastructure is just fine and the airport will rub along nicely as traffic recovers and corporate aircraft usage grows.
The bad news is that in the medium term the aircraft du jour will increasingly be A321neo - see Wizz, Jet2, Ezy, BA etc. These 235 seat aircraft need big stands and bigger runways.
For TUI & RYR its already the Max, which is smaller and less constrained.