The basic problem is that the HSTs (and I reckon the IC225s as well) are at the point where they just aren't really all that useful on the railway. Once IEP comes to the ECML, IC225s will just eat up paths and get in the way of the nippier IEP trains. Even on the MML, I'd expect them to be tripping up the Meridians.
Sure, the Mk3s are spacious and comfortable*, and perhaps newer trains aren't so much, but that's not the fault of the actual train. You could pack in hard, uncomfortable 3+2 seating into a Mk3 just as easily as you could do out a Meridian or an IEP with big, comfortable 2+2 seating, all with tables, all aligned to windows. I'm optimistic enough to hope that the IEPs will actually be fairly good comfort-wise.
I've alluded to this elsewhere, but I suspect the best use of HSTs is alongside IC225s on the MML, with the Meridians bolstering XC services, or possibly even used to run Paddington-Plymouth(-Penzance). Then, as we approach 2030, replace the entire MML fleet with new IEP trains.
*Comfort is subjective, but by-and-large, people seem to like the Mk3s.
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I don't know the route, but if there's a benefit to be had there, perhaps XC taking the Meridians to free up Super Voyagers might make sense. I guess it depends on how much it would cost to install TASS equipment on that line.
My understanding of the Cornish main line is that it's largely 60-75mph running with quite a lot of stops. It certainly doesn't seem all that suited to HST services. Then again, if you only really need the speed at the London end of the route, perhaps that's less of an issue...