Poor Wales.
Getting rid of old unsuitable trains to get some new old stock from the Underground.
Don't get me wrong, I applaud what Vivarail has done but TfW should have just ordered some 195s to cover them for the next 30 years.
TfW are being cleverer than you might think by ordering the 230s.
Firstly, their use on Chester-Crewe and the Conwy Valley is just a temporary arrangement to help with PRM modifications to the 150/8 fleet. (Likewise, the forthcoming use of 170s on Maesteg services isn't permanent.) Their real purpose if for the Borderlands line (Wrexham-Bidston).
There is a long-standing aspiration to make the Borderlands service more attractive: its southern portion is in a well-populated area, but the low frequency and lack of through services to Liverpool and Birkenhead are suppressing demand. Enough 230s have been ordered to double the frequency to Bidston, which should hopefully stimulate passenger numbers. We'll know after a few years of operation whether that's happening or not.
Through running to Liverpool is more problematic. You could run a diesel train as far as Birkenhead (or James St), but diesel trains aren't allowed along the Liverpool loop line. So you'd have to use either electric (if you electrify to Wrexham) or bi-mode trains to run Wrexham-Birkenhead-Liverpool services. Either one will be a significant investment to make. Thankfully, the 2tph service with the 230s will be able to show whether there is a business case for doing so.
The 230s are, as you say, "new old stock", and will have a shorter working life than something brand-new like the 195s you suggested. But given that the aspiration is to replace them with electric/bi-mode stock (if the passenger numbers add up) within ten years or so, that shorter lifespan dovetails quite nicely.
It's similar to the use of second-hand EMUs for new electrification projects: use cheaper rolling stock to improve the service and tempt passengers on-board, then you've got the numbers to justify buying new/longer/fancy rolling stock for the line.