I would agree that that they’ll now likely be on the route until the whole West Fleet is replaced, or at least the long distance stuff. Effectively from what I can see the west routes can be split into 2, local metro around Cornwall and the branches, Torbay and Exeter and around Bristol, then the long distance stuff such as Cardiff - Portsmouth/Penzance/Taunton, Weymouth services and also Brighton services. Obviously not enough long distance stock to go around hence why metro stock (turbos) are being used on 4 hour+ journeys.
Speculative, but if the SWR diesel fleet is replaced in the next franchise, will they go to razor blades, or could they be distributed across GWR and possibly Northern?