Regardless of the excuses or reasons for the 379s becoming surplus it is an ecological mistake to be prematurely scrapping any thing that has a carbon footprint resulting from its manufacture. Unless, of course, said item has huge costs ecologically connected with its use. A new diesel train replaced by an older electric or hybrid train might be justifiable.
At the end of the day it is ironic that, for the want of more electrification, we have relatively new electric trains going to waste.
The least worst approach ecologically would be to dispose of the older electric trains first. For example Southerns 313s and 455s.
It does not matter how cheap finance is these unwanted units still have to be paid for and the carbon footprint of manufacture does not go away.