The 27 sets Scotrail is taking are hardly 'a few'. When it comes to Class 43s, to operate a fleet that size you will need 54 of them, plus an allowance for heavy maintenance given the age of the things, say half-a-dozen, so that's more than half of GWR's 118 GWR Class 43s accounted for already.
Add in the 11 2+4 sets GWR (again with a few spare Class 43s) is expected to retain for an undetermined interim period while electrification and the resulting stock transfers are completed and that's 80-odd Class 43s that will still have a use for the time being once they are removed from GWR express services.
Ah, ha, you say, that still leaves 25 or 30 of the GWR Class 43s to play with, but there is a fly in the ointment in that 29 GWR Class 43s are fitted with GEC traction motors, which have always required extra maintenance, and have all been allocated to Landore depot since 2002 precisely so that its staff can keep a close eye on them.
It seems unlikely that GWR or Scotrail will want anything to do with the GEC traction motors if they can help it (and I don't think it's a coincidence that when you subtract 29 from 118, you get 89, basically enough to cover the two TOCs' likely requirements for Class 43s), so why would anyone else?
Similarly, in terms of being 'non-standard', all the EMT sets are fitted with VP185 engines, rather than the MTUs that were used to re-engine all the other Class 43s, so that immediately makes EMT sets less attractive to any other potential operator.
Which leaves the East Coast and GC sets with the MTU engine/Brush traction motors package that might be attractive to someone else, but that's about 15 sets in total, so maybe enough to bail out XC, if the Government would agree that it could have them in the first place.
Factor in the running costs of trains that were built between 1976 and 1982 and the question of maintenance, with expertise only likely to be available in the long term at Craigentinny and Laira - no one is likely to want to tool up a depot and train staff to maintain HSTs from scratch at this stage in their lives - and the options for future use are frankly pretty limited, beyond what we already know about.
The Scotrail sets have no guaranteed future beyond 2025, when the current franchise ends, and the GWR 11 are still subject to confirmation and have been described as an interim solution on a number of occasions by GWR managers, hardly suggesting they are expected to remain in use for a long time if they can help it.