Judging by the loadings on SV - Pontardulais , and the oft quoted Carmarthen to Aberystwyth line , I very much doubt it they could ever survived. The "Bont" local was often a single carriage train , with census returns (verified by research at the National Archives in Kew) , as less than double figures , hence early cutting back - even in the early 1950' - we all know about the Carmarthen line.
I wasn't talking so much about the local trains, although from what I've seen of old timetables the service was too irregular and infrequent to be of that much use anyway. I was thinking along the lines of the Llanelli - Pontardulais section being closed and the Bont to Swansea line being retained. That does leave the problem of Swansea Victoria, it's just that I've always been puzzled by the strange diversion to Llanelli, simply to access High Street station.
It's difficult for me to try and understand the reasoning behind some of the decisions that were made before I was even born. Some are more obvious, such as the Swansea - Treherbert route, where the decline of the coal industry must have had a massive impact. After all, the reason these lines were built in the first place was coal traffic from the valleys to the coast.