@cactustwirly is correct here.
We are dealing with the realities of where the law is and where responsibilities lie.
For example, we are entering a world where you could make a current train "compliant" by removing the existing small toilet - this may inconvenience 99% of passengers but (more importantly) it doesn't discriminate against the 1% of passengers who wouldn't be able to use it. Similarly, to use a Jacob Rees Mogg style quote,
no train is better than an inaccessible train (as far as the law sees it). If that means closing some lines or cancelling services because insufficient modern DMUs mean either running a Pacer or running no train then we'll end up running no train.
That's just how things are. Maybe, in an ideal world, we'd do things differently, maybe, in an ideal world, we'd have planned for the 1 January 2020 properly rather than desperately trying to upgrade everything at the last minute, but we are where we are.
A reliable eight coach train may be better for most people than an unreliable five coach train, but if the eight coach train discriminates against certain passengers whilst the five coach one is fully accessible then TOCs have to deal with that reality.
It's not just rail, it's the same in a lot of industries - it's better to provide something that doesn't discriminate than something that 99% of people may prefer (that discriminates against 1%). Our right to ride around on 1970s HSTs that have barely been refurbished in forty years and still retain period features like droplight windows doesn't come into it.
It's not about my opinion on what
should happen, it's about the law.