i used to commute (most days) from Southampton to London from about 1998 to 2008 and the trains were crowded then. half the passengers getting on at Winchester would not gets seats and would have to stand for the 55 minute journey to Waterloo. SW Trains (as it was then) said all the above points were the problems and it would take at least 10 years to improve things. More than 10 years later and things are no better. Nothing has substantially changed.
BTW Kingnut, they said the problem with having faster train was nothing to do the the 3rd rail electric supply but there were too many bends in the line were to allow faster speeds.
Southampton airport could be a great regional airport - its on the junction of two motorways (M27/M3) and right on the mainline rail line (Waterloo/Southampton and Bournemouth) but the runway is too short for bigger planes to use so it runs at huge under capacity. so very often, especially for holiday destinations, people from the central and south west part of the south coast have to drive past an under utilised Bournemouth airport (better length runway, poor transport access) and past an under utilised Southampton airport to go to Gatwick - who also say they need a new runway for increasing capacity issues. No overall planning goes into things. Maybe because airports are private companies and in competition with each other!
Re trains, you're right about that, but I didn't know the bends were the problem. The WCML got around that, partially, with Pendolino trains and that hacked a fair bit off times of the services to Brum and Manchester. I can't see that level of investment taking place south of London though.
The main problem, as ever, is cost. Going for overhead electrics just isn't feasible on the main line to Southampton and beyond. You can't make the trains go faster and you can't make them bigger. I think we're stuck with it, for another decade at least.
More generally though, the issue of ownership has been difficult. Privatisation was good and bad in equal measure, but the pandemic knocked the crap out of the rail business and set it back at least a decade. I know I keep banging on about this, but if you want good rail, you've got to be prepared to spend the money.
Re Southampton Airport, as I understand it, the runway length is the same as LCY. It can take jets like A318 and A319 and the Embraers that BA and Loganair use. I'm sure the airport would welcome more airlines using it, and I'm not sure it's a planning issue. I don't know if the issue is the way that airports are run, so much as it is about airlines not wanting to spread out a bit. But if Air France, for instance, wanted to set up shop at SOU, I don't see why they couldn't. KLM and BA are back there now.
it's such a great airport. Short walk to the station, single terminal, only a dozen gates. Fastest service to Waterloo is around 65 mins. Could almost be the sixth London airport.