Would agree it all comes down the net subsidy impact than it being older stock. If the older stock is less reliable and harder to maintain that can change the financial impact, the 350/2s are unlikely to enter that category.
I think it's unlikely the numbers work out for GWR to take them to displace Networkers. For GWR itself, unless more electrification happens (more cost) there isn't a need for 37 units. Some images online show them without bodyside cameras, would GWR require fitment of those? It's also likely to take two years to bring them into full service with the all costs associated with that including crew training. Maintenance costs would increase with two separate fleets instead of one being looked after at Reading. Longer term it seems likely battery units will displace some of these EMUs, the 387s can easily be absorbed by Southern given likely demand growth, the 350/2s won't have a convenient home.
For Southeastern, they don't have any 387s. If taking more 377s with Southern getting 387s it's likely difficult for Selhurst to continue maintaining all 68 377s on the Southeastern network. These 377s would need deploying onto metro routes requiring further driver training. The units are likely to have lower overall capacity than the Networker replacement fleet requiring more trains running for the same passenger capacity.
The headline lease cost might be lower for 350/2s than the Networker replacement, but all the other costs could easily counteract that.