It would be great to keep the 175's, but there's the big issue of maintaining them. The Mk4's won't be going anywhere for a while unless something drastic happens, as most of the carriages are owned by TfW now.
If maintenance could be transferred to Canton or a new Depot (Landore?) then the 175's would be ideal to keep West/HOWL and even new services Swansea-Bristol? But it all depends on if the numbers add up. The 153's won't last forever but there's currently no public plan to replace them.
But judging how there are 2 already stopped awaiting transfer back to the leasing company, it doesn't look like they will be staying.