The metrics of abstraction is an interesting one. Certainly the significant flow from West Wilts to Chippenham and Swindon is well known - indeed you may have noticed over the years that connections at Bath have improved from random to much better, specifically to help. If you take these people off the train into Bath Spa, you allow traffic to grow further into Bath and Bristol too without needing extra carriages, so it's more of a shakeout / adjustment than a straight abstraction in a flat use service, and an overall increase in services out of West Wiltshire which is needed often when you real there's been 80% growth in 10 years.
If you look at
http://www.wellho.net/3641_.html you'll see the timings between Swindon and Trowbridge via various modes, and you'll note how attractive the train becomes compared to the car - that's the real competition and, assured of a lasting service (we are looking beyond 2017 already - talk last Saturday to 2025 and beyond) you'll also boost Melksham, Trowbridge and Westbury as commuting towns - something which the various core strategies are planning for and anticipating.
I don't know of any "via Melksham" fares, so I don't see any significant loss of revenue. In fact it was only a couple of years back that we persuaded the fare setters to change certain "Via Bath" fares into "via Trowbridge". That got us away from the quirk of 56 pound single on the direct trains from Swindon to Salsibury, but less that half of that on the longer route and with a change of trains at Bath Spa. On the other hand, certain fares have been held down as an inexpensive gesture of mollification for very limited services, and I'm on record as actually encouraging some of these to rise to be less extremely cheap. That helps make the economies of the line work and will be a brick - just one brick - in the strongly built service that we're moving towards.
I hope that helps fill you in; I'm aware that I've not addressed some of the aspects raised, and some of those will have to be "wait and see". I've just got home from Manchester - too late for the last train at the moment, and I'm on the first train out in the morning headed for Patchway.
Edit to correct typo