More Stadlers would make sense if you were looking for fleet homogeneity - more cascade opportunity, potential for progressive conversion to OHLE/battery as electrification progresses (better journey times + potentially unit utilisation).
Rescue capability is a case of standardising couplers which *should* be possible.
197s may be decent for now, but are they really suitable for growth?
Where would you send the Stadlers to replace 197s? The answer is nowhere unless you start spreading them around the country where the only place they would be of any use where the 4 car capacity would be of any use is Chester - Liverpool and Chester - Crewe and even then its stretching it. The fuel range on the 231's is awful.
There's nothing wrong with the 197's. More 3 cars would be better currently but when the whole fleet is in there will be a lot more capacity on the majority of routes. The Cambrian is where the 3 cars are needed but that's a separate topic.
The flexibility created by having 2 and 3 car units is perfect for what TFW needs. 5-6 car trains on the trunk routes with the ability to split off and go separate ways on the quieter ends of the network. A 5 or 6 car formation is still more capacity than a 5 car mk4.
What would have been better for flexibility and consistency across the network is having other different configurations of 197s.
2 car - All standard, without the silly storage cupboard
3 car - All standard
3 car - Standard & Premium with the storage cupboard replaced by a decent sized kiosk where reasonable hot food can be made (Airline style but more presentable) rather than the full kitchen service.
That would be my compromise to losing the on board service and kitchen of the Mk4. Those 3 cars with Premium would always be run with another 2 or 3 car, so would be minimum 5 car with Premium and a decent on board service.