I don't buy this argument. Although the ideal was that both ends of a route should be under the same management, there were several cases in BR days where this was not true and it did not cause issues either operationally or commercially. Think Scottish Region and London Midland or Scottish Region and North Eastern (later Eastern Region).
I find your argument/comparisons here unrealistic. You ignore history. Even before the 1923 grouping, the LNWR had cooperated on a daily basis with the Caldonian Railway at Carlisle, as did the Midland and G&SWR, and across on the East Coast, the NER with NBR. This cooperation was only reinforced once these railways were merged into the LMS and LNER. I doubt anything really changed when it came to operations under BR - except perhaps some new job titles.
The Salisbury - Exeter transfer was totally different. The line was grafted - planted, if you like - lock, stock and barrel onto the WR, which had earlier only needed to cooperate with the SR at Exeter St Davids (and, yes, they cooperated on the Exeter - Plymouth routes).
Futhermore, the ex LSWR line was not Paddington's main concern - far from it. If you were in WR HQ on operations and you learned that, say, during one week, you had had four failures due to loco casualties on Bristol trains, four on Exeter/Plymouths, three on South Wales and seven on Exeter-Waterloo workings, which do you think caused you most concern?
I'm pretty damn sure that - unless by some quirk the PM happened to live in Yeovil or some such and his office had complained to the WR GM about things - it would not be the Warship problems on the former SR territory, even though they constituted the largest single group of failures.
I'm not saying it could not be made to work - indeed, it seemed to work, although perhaps after a fashion- but it added another interface that had previously not existed. The business world if full of plans that should work - but often end in disaster. We only need to think of a certain transport industry's 'privatisation' history to know that.