A true half-hourly express service Bristol - Taunton - Exeter would of course be wildly popular, just impossible to resource (even once all of the new Voyager trains are productive).
XC going to half-hourly would only make sense if the GWR Cardiff-Taunton service dropped out. In some hours the GWR is only 10 minutes behind the XC from Newcastle https://www.realtimetrains.co.uk/se...24-04-25/0000-2359?stp=WVS&show=all&order=wtt. Running Hourly Manchester-Plymouth and Hourly Cardiff-Exeter seems much more reasonable journey provision to me.Bristol - Exeter half hourly is probably of more interest to them than any other extension given the consultation a few years ago. Although as has been stated, this would require more trains.
Exeter isn't that big, and the demand to/from Bristol is reasonable but not 3tph Intercity all-day big. Maintaining connections to Cardiff (the GWR services form a half-hourly Bristol-Cardiff service with the Portsmouths, and that's very much needed). from Exeter is probably more important than having a 2nd XC every hour.