One of the problems that needs to be thought about is where do airport passengers travel to/from by train.
When look at Origin and destination, start to see places like Clapham Junction, East Croydon, Finsbury Park, West Hampstead Thameslink being popular. Are these real destinations (seems unlikely) or where people changed trains and tickets, possibly switching to PAYG or cheaper tickets (seems more likely).
Top destinations from Gatwick Airport station are :
London Victoria 2,727,742
London Bridge 1,084,056
East Croydon 955,931
London Blackfriars 570,213
Clapham Junction 557,459
(so 2-5 combined exceed Victoria, so is Gatwick Express a minority service to London, and actually Gatwick express is serving wrong part of London)
Brighton 352,748
Farringdon 317,224
London St Pancras International 298,967
(so another 600+k wanting different London station than Victoria, more evidence Gatwick Express might be serving wrong London station)
Three Bridges 132,875
Hayward Heath 112,589
Horley 79,470
(is this airport workers, or those dodging expensive airport parking)
Finsbury Park 74,325
Reading 69,600
City Thameslink 65,239
Eastbourne 61,147
West Hampstead Thameslink 60,863
(some of these suggest people are connecting or driving to get a direct train, I suspect part of these numbers are not actual other end of journey to/from airport, but simply place to pick up a direct train)
Horsham 58,920
Guildford 58,159
Redhill 56,665
Southampton Central 54,496
Balham 53,420
(only now do we get below 1000 journeys per week)
Hove 47,515
Purley 43,562
Cambridge 38,775
Wimbledon 38,054
St Albans City 35,426
Crawley 34,963
Chichester 32,505
Shepherd's Bush 32,364
Putney 31,246
Havant 29,820
Burgess Hill 28,930
Lewes 28,440
London Waterloo 27,907
Portsmouth Harbour 27,710
Vauxhall 27,702
(there's another 150k for London, but with journeys that require a change of train, so now got to start considering connections to trains serving the airport)
Polegate 27,412
Worthing 27,202
Fratton 27,118
Tottenham Court Road 26,239
Luton Airport Parkway 26,191
(will stop there at the 500 per week, in the list, if I had carried on as far down as 10,000 per year, would have got places like Oxford or Bournemouth included, places that require change of trains, or with journeys of over 2 hours)
So if anyone is serious about expanding capacity, have to start considering how the service should operate, it is looking like Thameslink destinations are more popular than Victoria, so should the airport trains be running to somewhere like West Hampstead or Finsbury Park, serving 3 or 4 London stations then running fast to the airport. Until now some of these have not really been thought of as important.
Maybe some others in the list (eg Southampton need to have a faster or more frequent service. Perhaps there is a case for diverting the service from Cardiff, Bristol Bath etc to Gatwick via Chichester, rather than Portsmouth. And if Oxford is important, why not combine Gatwick-Reading and Reading-Oxford services. To serve expansion, need to cater for the masses, not stay stuck continuing a service pattern from decades ago because do nothing change-wise is easy option.