I often change at Cheltenham as it can be quicker to get into Bristol and the comfort of a Voyager beats a small DMU. As for the majority of commuters? In Chepstow they get the 7XP to Bristol or drive, getting the train is just not really that viable for commuting when the bus is faster. From Caldicot (single T) you realistically have to drive if you want to commute to Bristol, especially now the tolls have been removed, removing the financial reasoning for taking the train. What is actually needed is the original Severn Bridge to be rebuilt (they're planning a road bridge in the area, not that hard to add rail) so that Southern Monmouthshire and Lydney can receive a direct service to Bristol, only then would commuting to Bristol become viable by train, especially considering how poorly located Temple Meads is for commuting (I used to do it and very quickly switched to the bus).
What's really needed is better connectivity to Severn Tunnel Junction (as the hub for the area), more parking, as well as more services stopping there.
For instance, when the M4/M48 junction was built they neglected to put in any new spur roads to the station (although they did put in a bridge and access road for the tool booths on the M4) , which could have enabled a faster connecting bus service to Chepstow using the M48, allowing Chepstow commuters more opportunities to catch the train to Bristol/Newport/Cardiff etc.
A junction on the M48 between Rogiet and Caldicot could also have been used by Chepstow (and east Newport/Gwent) motorists to reach STJ for trains to Bristol, reducing traffic over the Bridge and down the M4/M5/M32/A4.
I don't doubt if they increased the frequency of trains at Chepstow and Caldicot there would be more passengers commuting by train, although we don't know the number of commuters who can easily access those stations, so busses to STJ could probably have been successful.
