But leaving half a 350/2 fleet sitting idle isnt very cost effective either.
Because of the leasing structure in use, the railway isn't paying for the 350/2s if they aren't used. The reason they are not in service anymore is that, unlike the other 350 subclasses, they were more expensive than new trains. The ROSCO only has itself to blame for that.
I would like to see the 350/2s back in service, but it should be because they offer an improved service somewhere, for example by replacing worn-out/end-of-life stock, or expanding capacity or frequency. If the proposal exists solely to bring the 350/2s back into service, it almost certainly would be better to not do it.