Indeed, Eastleigh is big enough to support 2tph to London IMO. I know people can drive to Parkway, but then again why should they?
Eastleigh IIRC had 2tph fast or semi-fast to London continuously from 1990 to 2019, gaining the second train when through services towards Fareham started running. Even during the period the Fareham services didn't run through to London, it was served by the third Waterloo-Southampton service.
Plus, the xx39 also provides connectivity from the medium-sized stations in general, e.g. Farnborough to the South Coast, and of course provides 2tph fast from Farnborough to London.
Much of the SWR timetable has been quite a mess since COVID. Every year it seems to get more complex and less reliable, yet frequencies aren't actually going up. There are numerous factors at play of course (e.g. rolling stock availability, weather-based disruption) but a good amount of the issues can be improved upon with a better timetable.
For example, take a look at the line between Southampton Central and Basingstoke. Different stops are picked up by different trains (Micheldever and Shawford served by separate services), destinations vary by direction (Bournemouth-Winchester northbound, Winchester-Soton southbound) and the likes of Swaythling and Eastleigh remain with a subpar service given the local population. It's not terrible by any means, but attempting to understand the timetable if you live locally is a headache.
Thankfully the network is quite self-contained other than in a few spots (Southampton, Salisbury and Portsmouth) so there's enough flexibility to design a decent timetable without too many compromises.
The main 'checklist' additions for the 2027 rewrite, in the off-peak at least, probably are:
- 4tph Hounslow Loop
- 4tph to Epsom (doubling Dorking & Guildford via Epsom frequencies)
- Return of the hourly Southampton semi-fast (went to Poole pre-COVID, but I think the Bournemouth-Winchester is better suited for the New Forest local stops)
- 2tph Portsmouth Direct line semifasts
These are the outlying missing services, of course there's other routes that can be improved with a bit of tweaking. Don't want to pin everything on GBR (even if I should), but if these four key improvements are brought back then DfT-run SWR can claim it as an overdue success. If not even two of them are implemented, then it's likely to be indicative of GBR policy in the coming years.