I think they will only be a stop gap until a new train is designed that can operate the "Devon Metro" and Cornish lines.
A Metro is a very different kettle of fish to a rural branch line - anything ordered to work both is bound to be an inappropriate product for one or the other. Assuming 'Devon Metro' is an Exmouth-Paington all stations stopper then my feeling is that something like the class 166/165 units (but ideally with end-gangways if running in multiple - not sure how many coaches these load to) would probably be appropriate there. The rural branches though would probably be better off with a modernised, air-conditioned, version of a class 156 (which, apart from the rather higher top speed and lack of end-gangways, is kindof what the 175s are). Also, I hope any new stock will have electric drive (ie. diesel-electric with traction motors) and (at least passive provision for) a pantograph to allow the diesel engines to be turned off if/when running under the wires.
There are plenty of IETs working services for which they are an over-provision (and I don’t just mean “diagram fillers”). The problem is that post COVID there are (in theory - which is all that counts to the DfT bean counters,it seems ) too many IETs for the IC services they were built for, and nothing like enough dmus for the expanded/expanding local services around Bristol and the West.
The problem in my view isn't quite as simple as "too many IETs". Rather, the number of IET coaches ordered might have been about right, but too many of them were ordered with driving cabs rather than intermediate vehicles (ie. too many units overall, but more of the remainer should have been 9-cars and possibly some 7-car sets instead of any 5-car units).
The intention was always to use 387s on Cardiff crowd busters.
Yes, as 12-car formations if I recall correctly (presumably, until the wires to Swansea were cancelled, these 12-car 387 plans included the Port Talbot - Cardiff crowd busters that GWR used to run with IC125s?). Once you take off the larger cabs and kitchen space on an IET, the roughly 12x20m of a class 387 crowd buster (240m) could well have a greater length of usable passenger space than the roughly 10x26m (260m) of the longest IET formation.