I don't exactly see the big deal. It's 2023 - it's not really acceptable to be using rolling stock at higher speeds where there still exists the risk of someone physically falling out. I appreciate its a low risk, but it still exists, none the less.
As others have mentioned, various precedents have been set (Hastings unit, 121 etc). This has been long on the horizon, so if any operator hasn't atleast made preparations for the legislation/allocated funding, it's really more a reflection on them (especially when many smaller heritage operators have gone out of their way to enact changes well before the deadline).
There's various solutions to many of the issues, specifically, if power and train length is an issue, something like the hybrid BSO/DBSO generator vans used by Network Rail would be able to achieve the power requirements whilst also maintaining seating space requirements. That's not me saying to put DBSOs in the formation - simply that a brake van vehicle already in the formation could sacrifice its storage space for a generator, without eating into passenger capacity.
This may be a costly project, but I do rather think an expensive legal case would only kick the can further down the road, spending money on lawyers rather than making a change that will, at some point, need to be made.