The DfT has specified and procured trains for Thameslink, ICEC and ICGW, rather than leaving it to the TOCs, but now they say they are leaving organisation of where to cascade the older stock to the TOCs
I think that the current set up makes sense (and that ordering new stock is very different to fighting over spare stock for cascades).
For something as groundbreaking as IEP is supposed to be, it makes sense to have one big order of trains with common features - so that future infrastructure improvements can be based around this new 26m "standard" for new coaches.
If we leave procurement of new stock to individual TOCs/ ROSCOs then we run the risk of being lumbered with diddy orders of non-standard stock (e.g. First and the 175s, First and the 180s) which may be okay for the first few years of operation but can't be easily cascaded elsewhere due to the limited numbers. We could end up with more stock being built that can't work with similar stock elsewhere (e.g. Voyagers and Meridians) rather than one big fleet of similar units.
If individual TOCs want to argue the toss over what happens to "freed up" 170s/319s (etc) then fair enough, and I don't think that the Civil Service should be worrying about the small scale stuff - but at least we can have some kind of central control over new builds (and ensuring no corners are cut, that we get stock that is capable of working with other stock, that it's easier for follow on orders).
As far as the cascades go, I don't think that there's a huge difference between a newly electrified line getting a 317 or a 319 or a 321 - they are essentially different flavours of the same thing - and I think that the civil service has better things to worry about.
So, yeah, leaving it up to individual TOCs/ROSCOS to procure stock leaves the danger of (re)making mistakes of the past with small classes (180s) or classes that can't work together (220/221s and 222s)...
...but making the Government decide on cascades is either going to create lots of extra bureaucracy (worrying about which route gets which trains) or will be subject to lots of political argument.
(I should say that BR had its faults in this respect too - e.g. did we really need 141s and 142s and 143s and 144s? One megafleet of identically manufactured Pacers would have been better - albeit with some getting a middle coach)