You're making a leap too far in the logic.
The problem at the moment is that no-one makes a suitable diesel engine for use in DMUs capable of meeting existing emissions legislation. You can get around this by using an older engine and fitting it with particulate filters to clean the exhaust before it's vented to atmosphere. This the reason why Cl172s have all the bulky exhaust trunking around the bogies at the inner end of the cars. Unfortunately this means that the engines can't breathe properly due to the increased back-pressure that the filters introduce into the exhaust, so they have to work that bit harder in order and consequently they do use more fuel. If someone came along with a clean engine suitable for DMU usage they would clear up.
I think that there are other issues at play too. Chiltern did receive Cl172s to increase their DMU fleet, but there are restrictions on the construction of new trains. The easiest way of expanding the fleet is to absorb surplus stock from elsewhere and, in the absence of other suitable DMUs, that means locos, coaches and DVTs.
O L Leigh