The reality is that DMU's will be required long into the future, to a time beyond when the currently new DMU's are life expired. The DfT will have to bite the bullet at some point to purchase new rolling stock, because at this time, there is no alternative to DMU's, we can't electrify all routes, battery technology isn't advanced enough, leaving holes in the floor to work trains flintstones style isn't really going to be popular
Biting that bullet now, will ensure the lessons learnt from compliance with earlier euro emissions standards on rail based diesel units don't become lost, because once that happens, time gets reset to year zero and the lessonshave to be learnt all over again. The recent ATOC report on rolling stock sets out a need for upto 2000 diesel powered units beyond 2025 and 1500 beyond 2042 - giving a need to build upto 50 units per year to just satisfy ongoing rolling stock replacements.
I've not really heard a strong arguement for not ordering new DMU's yet - yes they will cost money, but realistically, just how long can the current fleet of 14x's and older 15x's keep going ? Someone has their head buried in the sand now and will end up leading us down the path of having to spend a larger amount later to reinvent the Eco diesel wheel.