Ultimately, sleeper trains are a luxury product and the price is what it is.
I am fine with the concept of luxury products - I never expect to be able to book a first class plane ticket for instance. If the berth is £100+ the majority of the time, stop claiming it’s £40 and getting our hopes up. Is my sentiment.
In relation to on the spot v. pre-booked pricing, I get that they’d rather you booked in advance, and I’m sure making the walk-up price the maximum is a ploy to direct you to do just that, should there be a next time.
As Deerfold says, if bargains can be had, regular users would probably get sniffy, stop booking ahead and start taking a chance on the night.
But in this day and age, I feel there must be a tech enabled middle way.
E.g. you could scan a code when you get on the train, then a few minutes later, receive a link offering you an ebay style auction on the remaining berths. It’s stressful enough so that you‘d rather book in advance, there’s no guarantee, but you get a “fair price” based on actual supply and demand in the moment.
Anyway, I just hope the occupancy is good enough to keep the service going. It looks like a fun option.