The thing that is odd here is that you are actually subsidising comparatively rich people (no plebs will be able to afford the sleeper). These people can and should pay the full cost of the service. The sleeper service should at least break even. Why on earth should poor folk subsidise it?
That is quite a philosophical argument. By any measure, the average rail user (not just sleeper user) earns considerably than average wage and is much more likely to be ‘ABC1‘ than the population as a whole.
Therefore it is fair to ask the same question for the rail system as a whole.
(The answer, of course, is the societal benefit it brings).