As has been said, despite being limited stop, these services weren't actually all that faster due to pathing, the short station dwells, and general nimbleness of the 377s, and so GWR diesels under the wires all the way to Brighton won't be missed, as long as the connectivity can be maintained.
I think the west coastway is a few platforms (unbalanced 'overtaking' platforms at Barnham and Worthing, etc etc, this has been discussed previously at length on other threads talking about this very service) off having a true 'fast' service that has both less stops and is actually beneficially faster.
Where these services were very crucial, at least pre-pandemic, was the morning inbound and evening outbound from Brighton, where they filled an important gap in the pattern - usually full and standing with local passengers, and so significant dispersal when they didn't run (not unusual). I've witnessed both of these services leave passengers behind at the first/last stops into Brighton.
I think the west coastway is a few platforms (unbalanced 'overtaking' platforms at Barnham and Worthing, etc etc, this has been discussed previously at length on other threads talking about this very service) off having a true 'fast' service that has both less stops and is actually beneficially faster.
Where these services were very crucial, at least pre-pandemic, was the morning inbound and evening outbound from Brighton, where they filled an important gap in the pattern - usually full and standing with local passengers, and so significant dispersal when they didn't run (not unusual). I've witnessed both of these services leave passengers behind at the first/last stops into Brighton.