Somebody is paying for this largesse, and it ain't GWR.
Certainly the ScotRail fleet is well below the originally planned utilisation. Both have mitigating circumstances where either the TOC has not had the fleet for that long (ScotRail) or they are not long transferred to a different depot (LNER sets to Neville Hill). What's Laira's excuse?
There’s no excuse, it’s the same as the 91s at Neville Hill, getting parts for obsolete fleets is challenging. I don’t recollect just prior to Covid when 91 and mk4 sets were at their historic home of Bounds Green availability being much better for a similar number of diagrams.
It’s worth noting Scotrail rarely achieve their planned availability.
Power cars are so warped these days cabs aren’t readily interchangeable like they used to be etc.
All these fleets however have a much higher availability on the traction side than the Swiss Re 425 as per your profile photo or Re 460s (which are more modern than both 43s and 91s), indeed the 460s have 87 diagrams out of a fleet of 119 locos - 73% for an electric fleet which you would generally expect to be higher than a diesel HST power car. Swiss Railways are often said to be some of the most revered in Europe!
So I ask again what is unreasonable about the currently planned GWR availability requirements, they seem pretty good.