You have to wonder why previous operators got rid of them when they hadn't even completed a half life of service.
Arriva Trains Wales (ATW) didn't get rid of them, and I'm not sure First North Western (FNW) and TPE really did either. I probably still lived in England back then and was certainly not aware of the 175s before they started to appear in South Wales, but wasn't it a case of franchise boundary remapping, with the North Wales Coast Line (together with the 175s to (help) run it) transferring from FNW into the Prism/NatEx. Wales & Borders Trains franchise? I
don't claim to have any more than a layman's / interested bystander's understanding of either the process or the politics that surrounded the end of the ATW franchise and the award of the 'Operator & Development Partner' contract to KeolisAmey which (trading as Transport for Wales Rail Services) replaced ATW from late 2018. All I know is that TfW promised to replace the entire ATW fleet; not just the 175s but the whole lot (except, perhaps, the class 67 locomotives, although TfW today uses higher-numbered 67s than ATW's original trio (67001-003)) - maybe that helped them win the contract?
Regarding 197s. Why did TFW think they needed them?
TfW needed a replacement for the 158's so they decided to replace 158 & 175's with a single fleet which makes perfect sense, shame they bought junk to achieve it and didn't provide suitable depot capacity but there you go.
ATW/TfW needed something to boost capacity as well (far more urgently than replacing the 158s) and ATW already had all 27 class 175s, which were typically run as single units (ie. just 2 or 3 coach trains) on busy routes such as Manchester-Swansea, because they had such a limited fleet. The 175s are unable to work in multiple with other types, so there
is an
argument (not necessarily a sufficiently strong one) for replacing them in favour of a uniform fleet rather than just supplementing them. I still think supplementing them (and, in hindsight,
not with brand-new DMUs, but with bi-modes or cascades from elsewhere) would have been the better plan.