The 387s for GWR were a fairly late addition to the electrification plan. I'm sure I remember as late as the end of 2015, the plan was still to have a mix of 365s and 319s. Had this tender happened a bit earlier, we could still have had the 365s, but they've all been sent to the scrapyard now.
As an occasional GWR customer, it's a bit annoying to have new trains replaced by older ones! 379s are not much older though. Hopefully the 379s will get a minor refit for power/USB/WiFi, if that is what happens. Reading TCD shouldn't have too much trouble adapting, but I don't know about the similarity of drivers' cabs. Also, Wikipedia doesn't list 387s as one of the options for multiple working with 379s, which could be important for the transition period between the two fleets - but the source cited for 379 multiple working is from 2012, possibly pre-dating the existence of the 387s.