Networkers haven't been built for over 30 years (if you ignore the similar, but not identical 168s)
But recent diesel classes eg 196, 197 are likely to be around into 2060s.
The question for GWR is what does it get for Devon and Cornwall local services, even if electrification gets extended to Frome or Weston-super-Mare areas, there is no short or medium term likelihood of it getting much further, at best might get to Taunton, and even slimmer chance of it getting to Exeter area. Therefore in next decade no point buying battery EMUs for any services running south west of Taunton. Much more likely to electrify northwards towards Gloucester and Bromsgrove before considering going towards Devon.
Any battery EMUs are going to have to run to Dorchester, Redbridge, Worcester etc. And the longer distance versions are going to also need third rail 750v fitted