Looking at how long the 150s have lasted, the 175s should be good for rather longer than 5 years yet, more like 15-20.
From my point of view (ie. a passenger,
not rail staff) 442s would have been more fit-for-purpose on WAT-PMH fast services than 458s and a ScotRail HST is more fit-for-purpose on the Inter7City network than 158s or 170s (although economically, with two power cars and an excessively high top-speed a hypothetical 5-car class 175 (no intermediate cabs) would probably have been a better choice).
Returning to the topic at hand, class 175s would be more fit-for-purpose than alternative stock (in brackets) in at least the following suituations:
- in pairs on GWR's Cardiff-Portsmouth service (instead of 165s and 166s)
- in pairs on Northern's Leeds-Carlisle*, Chester-Leeds and Blackpool-York (instead of 195s - * not sure if they're used there)
- in pairs on XC's Cardiff-Nottingham route** (instead of 170s) ** not Birmingham-Stansted due to Sprinter differentials
- on TfW's Conwy Valley, Heart Of Wales, Swansea - Milford Haven, Swansea-Fishguard and Swansea - Pembroke Dock services (instead of 197s and 153s)
- on various new services, which would be desirable to attract modal shift from road (eg. Cardiff - Carmarthen/Pembs. express services via the Swansea District Line, where the 175s would be far more suitable than Cavities).