XC currently have 29 170s - 7x 2 car and 29x 3 car, so taking on a fleet of 27 175's with 11x 2 car and 16x 3 car would not only mean less units but also a big drop in carriages. So they'd have to take on the 175's and keep a smaller fleet of 170's, cascading some to another TOC to see any benefit.
It is a shame TFW couldn't find somewhere to maintain them and keep them on the network. There's a lot of grand plan in Wales for various metros, namely North Wales, Swansea and South Wales (Cardiff-Newport areas), which TFW won't be able to manage any further growth with their current fleet.
The 175's could happily worked the Heart of Wales (4 diagrams) and Swansea - Pembroke Dock (4 diagrams) services, could have continued working Maesteg - Cheltenham (5 diagrams) and Ebbw Vale lines (5 diagrams) until the 231's move over and still had spare capacity to pick up other work such as Swanline & Fishguard Harbour.
Although the 153's are workhorses, they are reaching end of life and there's no plan to replace them currently. Although the long term plans will see fewer 153's in the fleet, there is currently 26 aged 153's being kept going vs 27 175's.
If any of the grand metro plans go ahead, then TFW will need to order more units.