The is an advantage for the long term. With a single fleet it unifies maintenance and fault management. It will allow for greater operational flexibility, reduce staff training, reduce infrastructure demands and potentially introduce network wide DOO.
395s are uniquely placed and will always remain as independent and Networkers need to go without a doubt. If your going to split Metro/Mainline Then you should bundle 376s in with a fleet change for Networkers. This is an opportunity to bring an entire fleet up to modern standards. Toilets, Aircon, WiFi, plugs etc etc. If you fast forward another 10yrs we could end up in the same situation again, where part of the Metro fleet needs replacing (the 376s) and because of whatever franchise system exists new fleet replacement gets constantly kicked into touch.
Also consider a future where ERTMS is being introduced, CSDE, SDO, ASDO, FASDO, ABDO, ABC, 123 etc. are becoming the norm and a new fleet would speed up the rollout of balise introduction and push new signalling forward. Again, if this is all happening, then we should take the opportunity whilst its available.
I'm sure there is a financial benefit of ordering an entire new fleet. With the 707s, isn't there an issue with the costs involved ? Where is was cheaper to order a new fleet with lower leasing costs. Dumping 707s onto a franchise purely because they are sitting about doing nothing feels counter productive and a penalty to whoever is forced to take them.