I think that is where the 350/2s could eventually come in. They are available to meet growth if they are eventually needed and can displace the 379s but in the meantime, the timetables need to be resource led.
The 350/2s are fast becoming the new 365 or 442 - basically a fleet where 101 ideas, most of them crazy, keep cropping up.
Why would GN want to take on the 350s ? It creates a driver training and maintenance headache which they've recently got rid off by standing down the 365s. The 387s and 379s are sufficiently similar that the training and maintenance headache wouldn't be reintroduced.
They, perhaps, would have made more sense for EMR Connect, offering the gangway end which the 360s don't - but then that would have raised the question of where the 360s would go.
I am not sure about the Southern region, but when you look at GTR as a whole it is very possible they could cascade out the 455s / 313s.
When you look at the companies, Thameslink - dedicated fleet with lots of spare capacity. I guess Cambridge - Maidstone East isn't relevant here as it would be South Eastern units it would be replacing. But we have 12 car trains running between London - Cambridge / Peterborough, so running 8 cars on other routes (instead of 12) may be enough in the peak. Cambridge passengers may be unhappy admittedly that they have a choice of a seat on the slower Thameslink service or need to stand on the Great Northern. I suspect that these arguments can be debated South of the Thames, If London - Brighton has lots of 12 car trains via Thameslink, does Victoria need them?
You mention 10 car trains, but could 8 suffice? with the drop in passengers numbers meaning the 3 car trains can be cascaded elsewhere.
I know it is often short sighted doing this, as when traffic grows again you can't build new trains quickly enough, but as you can't shorten routes with Thameslink trains, shortening the others and encouraging people onto the excess capacity of Thameslink may be the way of retiring the 313s and 455s. It is odd we are talking about reducing train lengths, I suspect dft would rather we drop the frequency to achieve the same thing and keep the train lengths the same, but that may have a hit on the revenue.
There are 30 x 313 and 46 x 455 - a reshuffle might just do away with the 313s, but it would service reductions / shortening formations remove the 455s ?
I'm not sure 387s are the right answer for the 'inner suburban' type work the 455s do in any case - I still think a small order of 707/717 type Desiro Citys or 710 type Aventras would make more sense.