It can’t be acceptable to scrap non-BREL era DMUs yet when it’s unacceptable to order more.
In a perfect world I would agree with you.
But in the climate of the railway that we are in today, in what way is it not acceptable? If you are talking logic, you need to look firstly at what the need is, who benefits, and at what cost.
A major consequence in how the railway has been run post privatisation is the relatively conservative procurement history of new build fleets. They used to come in largely on the basis of replacing older fleets that are on their mechanical deathbeds, rather than procuring with forward thinking in mind based on the needs of staff and passengers and the company at large.
Sooner or later by continuing this approach you hit that juncture where you need to grapple with the situation at hand and cut your losses. Every medium/large TOC has either done it already, or will be forced to do so relatively soon. Look no further than the mess Northern was prior to the CAF order, or GA prior to the Stadler/Bombardier orders. Or look at the situation across Inverclyde for ScotRail where you had six different fleets turning up to work the same services at one point until 2019.
The fact is that small fleets such as the 175s are a hinderence within TOCs who've had to procure new builds (see reasons in my last post as to why). It doesn't matter in this regard how old or new the fleet is. If they aren't needed, they aren't needed.
Ultimatley, it's proving cheaper in the longer term to buy new than it is to lease and maintain something that's already built.
Why bother?
It must be logical to retain as many 175/185/195s as possible until widespread battery/hydrogen/new electrification materialises. Diesel isn’t obsolete until all the wires are up.
Not strictly true when you remove the politics behind these things happening.
Battery technology already exists that allow you to charge up batteries while under the wires, then use the stored electric on non wired lines. As we are seeing in Scotland, by 2030 there'll be periodic blocks of wires for this purpose so as to avoid diesel running.
Though again, political will depending as well as the technology catching up that allows longer range off-wire running.