You'll find plenty of posts in these threads talking about ROSCOS "ripping off" the railway in the past, and this is the DfT getting its payback.
That's why 350/1s are staying and 350/2s are not.
That was in the early privatisation years the ROSCOs, especially Porterbrook (but they all did) charged high leasing rates to ToCs for old BR era stock because ordering new was expensive and they could charge and ToCs had no choice but to cough up and bill the DfT for subsidy. Ironic how ROSCOs profiteering off the DfT came out of a system the private enterprise idolising new Labour and Tories created.
The order for 379s at the time was partly political with the threat of Derby losing jobs otherwise I'd imagine they'd have ordered a few more 350s instead to go with the existing 360s.
National Express did not want to invest anymore than the bare minimum required to keep the franchise. They had an obligation to make that order for the 379s as part of a network improvement plan that they needed to keep the franchise from 2008-2012 in response to a white paper published in 2007 to improve the quality of services on the WA.
This is probably partly why Akiem has held on to the 379s for over a year now even though the DFT won’t let anybody have the 379s.
It's not that the DfT won't let anybody have them, a ToC is more than welcome to have them, the DfT just won't be willing to give out a subsidy to cover the costs of running them. If you found a profitable OA operation to use them on that would be different.