I'd be more likely to be convinced if you could identify any other route serving similar populations with similar characteristics (parallel route linking both ends, negligible onward journey opportunities either end) which operates a successful train service.
Apart from the impossibility of finding two routes which are geographically identical, I disagree with your assertion of there being negligible onward journey opportunities, particularly if Bath trains were continued onward to Bristol.
There isn't a parallel route linking both ends, without a change of trains and a deviation in direction (as you yourself identify). Your whole argument seems to rest on the presence of this "parallel" route, which is supposedly so much quicker and more convenient than the direct route would have been as to sap any demand for travel between the Bournemouth/Poole conurbation and Bath/Bristol, which I find unconvincing.
This is why I see the the East Suffolk line as probably the closest in similarity, although passenger services on other longish routes such as the Settle - Carlisle could also be seen as similar in many respects.