The 387s brought better facilities to the relevant GN routes - eg power sockets, air conditioning, faster acceleration, tables etc. Should GN travellers have been deprived of these facilities despite the stock being available?
The performance difference between a 365 and 387 is minimal, indeed GTR never bothered to amend the timings to differentiate between 365 and 387. As for power sockets, the 387s may have them, but the 700s most certainly don't, and for that matter nor do they have tables, which the 365s do.
Yes, it is shame that 365s couldn't displace the 313s but the reasons for that are well versed.
It is inexplicable that the new found slack in the 377 fleet hasn't been used to eliminate the 313s since March 2020, let alone 387s occupied elsewhere, and that service cuts aren't being introduced on the Southern network to release more 377s to eliminate the 455s.
I understand there is, in fact, a tentative plan to bin the 313s. Likewise there is certainly a tentative plan to do something with the 379s. May even be related!
The problem comes should the GN peak extras return in their pre-2020 form. I'm not sure there will be such a surplus of Electrostars then, especially if Gatwick Express rebounds (and remember Gatwick Express isn't just about Gatwick, but about providing peak-hour commuter services from/to Brighton).
Not sure why C2C would have wanted a limited number of 20 year old non-standard EMUs either ? Far easier to do what they have done and borrow 387s which are much more similar to the 357s they have than the 365s.
The rationale for c2c is no SDO issues, no DOO monitor issues, and only a couple of years older than 357s. Run with them as an "inner suburban" fleet (for the Tilbury loop), and then replace the whole lot together when the 357s are due for renewal. It may well also be the case that with the LTS lines having modern signalling, any compatibility issues would have been less compared to elsewhere.
I'm not saying it would necessarily have been an optimum solution, but it would have provided a viable utilisation for the 365s stored in 2018. It would also have brought extra capacity to c2c much sooner than waiting for new build.
It wouldn't have been my number one utilisation though. All 40x 365 retained on GN would have worked much better.