No. The route was transferred from Intercity to London and Southeast Sector shortly after Sectorisation in the 1980's
Southern Electric Group write in
FROM 10202 TO 50050: WATERLOO - EXETER DIESEL OPERATIONS
"The 1970s proved to be a decade far from retrenchment. By 1973 the line ranked amongst the top ten BR services based on revenue generated per coach mile with steadily increasing load factors but no real improvement in service level or capacity"
This is for the Waterloo - Exeter service, which would include a proportion to revenue between London and Basingstoke, and virtually all of it between London and Andover and Salisbury. In 1973, (and for some years afterwards) the Waterloo-Exeter service were the only through trains between London and Salisbury, augmented by a two hourly stopping service from Basingstoke to Salisbury connecting with a London train. It can be expected that the Waterloo-Exeter service took the lion's share of the traffic and revenue and would be crucial to the finances of the route. At this time Salisbury and Andover had, by far, the least frequent London service [for that distance] of any comparable places on the Southern Region.
Salisbury is irrelevant as it is the line west of Salisbury that is at issue (and would have shut if certain people had had their way).
On the contrary, it is all about Salisbury (and its environs). Taking your proposed semi-fast Paddington-Exeter service versus the semi-fast Waterloo-Exeter route. From London, the stops at Reading and Newbury are roughly equivalent (traffic wise) to those of Basingstoke and Andover on the Waterloo line. West of Newbury the Paddington train would stop at multiple stations the equivalent of the Tisburys, Gillinghams and Crewkernes of the Waterloo line. However, the Waterloo line also has the large places of Salisbury and Yeovil to provide traffic. On the Paddington line, Taunton can be discounted as the bulk of passengers would travel by the other express trains.
Neither service had or would have had any intermediate traffic to speak of, except to and from Salisbury of which there was no equivalent on the Paddington line. Yeovil people would never have gone by car to catch a train at Castle Cary in 1964, or even now - the London train service from there was no existent then and not much better with the proposed semi fast service.
As you rightly point out, the Salisbury-Exeter section was nearly closed. If the Waterloo-Exeter trains had not had Salisbury to call at, the whole line west of Andover would have gone the way of the Somerset & Dorset!
The financial situation on day I is the same, but good management can rectify the situation with one scenario but not with the other.
No, the situation on Day 1 is much worse. In the closure scenario, the staffing of some stations would be immediately saved, and the movement costs of the infrequent trains. Some revenue would have been lost, but this certainly wouldn't have paid for that. In the semi-fast scenario, some of the revenue may still have been lost (the two hourly clockface timetable not coinciding with the local travel needs) and a bucket load of extra movement costs (as the service being replaced only ran sporadically) and precious little generated revenue (how many Wellington to Bruton tickets sold on Day 1?). No chance for 'good management' to gradually win revenue - they would have been sacked in short order and the service soon cancelled.
Getting back to Soham, I can't help thinking of a SW parallel being the campaign for Sparkford station to be re-opened, for the campaigners then to discover that they would get no direct London trains......