One might also reasonably argue that as Government already subsidises the travel of thousands of daily passengers between Scotland and Euston every day, that the sleeper should be directed to minimise subsidy overall.
If the number of carriages remains the same, which is my assumption, the subsidy would largely remain the same. From your perspective, the desire is therefore to get more revenue from thirty lie-flat seats than from ten compartments. A lie-flat bed probably ought to be more expensive than a conventional seat, but definitely cheaper than a sole-occupancy compartment. Obviously dynamic pricing complicates matters. But in big handfuls, we can probably establish the bands that would be revenue neutral on the busiest and the quietest nights.
The minimum income (i.e. all single occupancy at the lowest price) from a ten-compartment sleeper is £1,790. If you can fill thirty lie-flat beds at a minimum fare of £59.67, you're up on the deal. The maximum income on the same basis is £4,540, suggesting that the upper bound price for revenue-neutrality is probably £151.34 or thereabouts.
That gives us a reasonable indication of where the revenue-neutral price range lies. It's not obviously ridiculous. In fact, I suspect the fares could be somewhat higher, since the upper bound undercuts the Anytime Single from Inverness to London.
If there were a serious possibility of doing something along these lines, then the fares would of course be worked up in more detail, along with an assessment of how many sleeping cars should be replaced with (well, reconfigured as) lie-flat seat cars, and whether or not people in the lie-flat seats should be permitted to use the lounge car.
Since there's no such possibility, this is of course all entirely academic.
Put another way, no point spending more cash to attract more people to the sleeper who are going to be paying less for their tickets if it more leaves empty seats on Avanti that are already paid for.
It's certainly possible that increased sleeper capacity might cannibalise day trains on the Lowland routes, though the level of competition (Avanti, LNER, Lumo, and air travel) does suggest that an extra 40 people a day in each direction would be a drop in the bucket to other providers.
On the Highland routes, the day trains require loss of a full day to travel (Inverness, Aberdeen) or aren't at all credible (Fort William), and additional, affordable capacity on the sleeper would be invaluable.
If, on the other hand, the Scottish Government has formed a view that the role of the sleepers is to facilitate rich tourists coming to Scotland to spend lots of money on whisky and shortbread, well, they're probably going the right way about it. That doesn't mean that the people of the Highlands have to like it.