But the *circumstances* are the problem.
If I go to a hotel at the top end of the market and I find that when I go to take a bath in the evening, there is no hot water in my room, then the hotel might offer me to change rooms. But nobody wants to have to shift all your stuff you just unpacked, or expect to have to do that, at 9pm. One might imagine the hotel offering some sort of recompense for this inconvenience, which was nothing to do with me, and perhaps comp my meals, or offer money off a future stay, or a partial refund, or something at least.
I don't think saying "oh well, never mind, best that could be done" is the best that one can hope for when their own train failed. "Free travel" to your ticketed destination? Um, that's what I would have paid for in the first place! That's a contractual right in the circumstances, not a customer service action to redeem for the inconvenience.
The Cally is not some utilitarian EuroNight service but actually Britain's premier train. It can regularly cost you £200+ for a room. The top rooms cost upwards of £400. It is not cheap, and is at the very, very top end of all regularly scheduled public trains, not just in the UK, but in Europe. The prices and service pitch come with the attendant expectation that kicking you out at Carstairs and making the rest of the trip on a Voyager - which I repeat, is a contractual requirement - is far from ideal and should come with some customer service recovery.