You've not quite answered the question I intended - but on your points:
You say that the air and bus services are operated on "commercial terms" but that's not really true. Buses need a publicly funded road system, amongst other things, to operate.
But how about air services: as far as I can make out highland airports recieve a subsidy of about £25M per annum. That puts your figure (based on I'm not sure what) for the marginal cost of the sleeper, of £4M, in context.
The socio-economic reality of transport, amongst other things, in sparsely populated areas, is that it needs substantial publicly funded subsidy in order to function as a public service; I'll not argue with that. How much that subsidy should be, of course, is a political question, and politics are part of socio-economic reality.
You'd previously implied that the Fort William sleeper is not 'viable'. Well, it wouldn't be viable as a purely commercial enterprise, sure. But it is viable if there's the political will to fund it. Same would apply to the Wick service.
I'm not going to argue that the wick proposal necessarily makes sense. But I think it's hasty to write it off as an idea just because it would need a hefty subsidy. That doesn't automatically make it nonviable. By the same logic the orkney air services and the A9 are not viable. I assume the ferry as well?
The service's value would not only be reflected in the revenue that it took in.
An interesting thing that came up in discussion on another sleeper thread, if I remember correctly, is that if you look at Caley sleeper's subsidy per passenger-km is actually lower than some of the regular english franchises including some commuter ones. Whether this is relevant or meaningful is tricky and ends back in political perspectives, again part of socio-economic reality.
I think it's easy to rubbish this Wick idea on the basis that it's "unrealistic". But what is "realistic" is no clear cut thing, as I think you are trying to make out. It's easy to say that the idea is outside of socio-economic reality but it's not as simple as that. If anything, the survival of the Fort William service proves this. Probably so do the various, rather expensive schemes to provide tiny hebridean islands with RORO ferry services.
Just a point on the HIAL subsidy. That is spread around all 11 airports - one assumes that Inverness could sated on its own commercially, but that Benbecula, Barra and Wick, amongst others, are those that consume the cash. Nevertheless, aircraft and passengers still pay pretty substantial landing fees. However these evidently aren’t enough to pay the full costs of the infrastructure
In this respect it is almost identical to track access charges. The annual cost of the imaginary sleeper service would include track access charges. However, as we know track access charges don’t cover the full costs of the rail infrastructure - not by a long way. (ORR data suggests that a further £312m subsidy is given for infrastructure in Scotland, on top of a similar amount granted to Scotrail).
The annual cost I quoted does not cover this ‘hidden’ subsidy to the infrastructure operator, so I would argue it is a relevant comparison.
The sleepers aren’t viable on a cash basis and never will be. The Scottish Government chooses to subsidise them (to the tune of approx £100 a passenger), because it judges that the wider economic benefit is worth it, principally by importing high spending tourists into Scotland, and enabling individuals of high economic activity to be more productive in travelling between London and Scotland. Part of this calculation will also be the pain and reputation all harm of trying to remove the sleepers. Serco is also subsiding the sleepers by around a similar amount through their losses on the CS. They are doing this as they are under contract, and must have calculated that it is more financially rewarding to continue with the losses and attempt to reduce / eliminate them than to break the contract.
The imaginary Wick sleeper is different in two ways:
1) the size of the market is very much smaller than that of the Anglo-Scots sleepers, as I mentioned previously. Rather less than 1/25th the size. Clearly this means that the number of high spending tourists will be similarly smaller (even assuming that such tourists are as interested in seeing the far north and Orkney as they are in seeing Edinburgh and the Highlands, which I strongly suspect they are not). And the number of ‘high economic activity’ individuals will be similarly smaller, again assuming that the proportion of these people wanting to travel to the Far North and Orkney is the same as those wanting to travel between the U.K. capital, the Scottish Capital, the largest city in Scotland, and the Oil capital of the U.K., for example (which, again, I strongly suspect they are not). Taking this all together, the potential wider economic benefit of such a sleeper is certainly going to be less, per passenger, than the current sleeper. Significantly less by my estimation.
2) the scale of the subsidy required is substantially higher than the existing sleepers. By my bag of fag packet estimate, at least 3.5x more, and quite possibly 10x more.
Therefore you would have to spend substantially more per passenger, and get significantly less benefit back. That is socio-economic reality. Given that questions have been asked about the continuation of the Anglo Scots sleepers in the not very distant past, and no doubt will be asked again at the next franchise renewal, it is simply not practical for such a proposal to get past first base. Particularly as alternatives would be far cheaper, and better.*
Finally, yes the CS has similar subsidy per passenger km as the big three regional TOCs - Scotrail, Wales, Northern, but this is not comparable. Other long distance TOCs are effectively subsidy free.
* I know someone who owns a private jet company, so asked a few questions. It would be comfortably cheaper to hire two helicopters and their pilots, 6 days a week, to operate an on demand service between the central belt or Inverness and the Far North / Orkney. Of course this might annoy Loganair.