I think the sleeper is unfortunately trapped in a kind of political dead zone where it is kept running because cutting it would cause an outcry, but at the same time there's not a strong enough will to really make it work the best it can.
That is, accepting it's going to need a subsidy but getting the greatest possible value out of that subsidy. They just didn't try very hard to offer a couchette-level option when the new trains were designed. Also, if the trains are in the position of selling out on certain nights then yes one option is to put the fares up even more but another is to consider running more beds per night, as has been discussed previously on this thread.
But adding extra services is not appealing to politicians who want to keep their heads below the parapet. It would likely involve arguing that a rise in the total subsidy could result in a lower subsidy per passenger.
Although I think a couchette-level option is the big missed opportunity, I also wonder if the seats could be better used given a bit of effort. That is (ignoring other constraints) whether, given some strong marketing and promotion, 2 or 3 carriages of seats instead of just one, could be filled on certain services at certain times of the year. I assume this would not be attractive to an operator from a revenue point of view - it would be a matter of maximising the social benefit of the subsidy. I don't think many potential customers even know the overnight seats option even exists - I never see them marketed as a thing in themselves, and the option often doesn't show up in journey planners (for example, the Scotrail app will tell you a single journey from Inverness to London overnight costs about £200 rather than the £55 or £75 that the seats actually cost).
(A couple of people will likely pipe up saying the seats are a waste of time as they'd choose a cheaper overnight bus instead - fine, they are not the best option for everyone, but they are for many people: they do have a number of advantages over the bus for certain passengers.)