This situation exists because no-one is willing to order new diesels. I think this is down to a combination of government unwillingness to buy trains for routes receiving lots of subsidies, and uncertainty over future demand for diesels caused by the enthusiasm for electrification and potential for battery powered trains. The surplus of EMUs will last only until the 317s,319s,321s are scrapped, which will now inevitably be long before the DMUs of the same age.
However at current prices I'd expect new fleets of diesels to be far cheaper than electrification. The day the government finally gives up on electrification may actually be a good one for passengers on unelectrified routes, if it allows operators more certainty in planning for the future. Someone will have to bite the bullet in the not too distant future - the Sprinters won't last forever.
It is a bit like why does nobody build gas fired power stations? Far cheaper than renewables but government has rigged the market against them so without guarantees nobody will build.