I'm a periodic traveller between London and Aberdeen - by air. I stay in a hotel there overnight on arrival.
It's very easy to get to the airport and park there for some days. Not so at Kings Cross. Most of my colleagues travelling to Aberdeen start off from nowhere near Kings Cross. Even if you do leave from a Central London office, just what do you do, with luggage and on your own, between 6 pm and 10 pm.
I would not send my female staff to Kings Cross late at night to wait for a train. One of the upsides of airports is their freedom from drunks, beggars and pickpockets in a way that main railway termini (and any associated car parks nearby) late at night have never been able to manage.
The two key downsides of sleeper travel are the jolting of the bed all through the night, and the non-existent shower etc facilities.
There is a long tradition of such services being operated to suit the railway. It would be nice to board a 10 pm departure by about 8 pm, if not 6 pm (like a hotel), and it might even be advertised as such, but you can bet it would be opened up 10 minutes before departure. Likewise arriving passengers have always been thrown out at 0730 to free up the platform for the morning rush. To ensure this, attendants traditionally go round and wake everyone up at about 0600.
The separate portions of Scottish sleeper trains and the characteristics of the couplings means you can only join them by pushing at about 5 mph with a locomotive controlled from several carriages away. The resulting big thump wakes everyone up.
They would be targeting the business market to/from Aberdeen. Ie. The oil business. Fares would be pitched higher than the normal unrestricted First Class fare but just below the airlines.
Air fares I pay are typically much less than unrestricted First Class rail fares.
The full unrestricted oneway fare on British Airways to Aberdeen is £378 and many firms are prepared to stump up for that for whereas the equivalent first class fare on Virgin East Coast (If I'm looking in the right place) is £230.50.
I've just looked at typical dates/flights I take on BA and the fare is £287.77, £100 less than you state. Meanwhile an "advance" first class on the daytime train, same dates, is £334.65, and an "unrestricted" ticket is £455. This is for a first class seat in a carriage which holds 48, which can manage the round trip from Aberdeen to London and back, and its associated revenue, within the day. You are proposing fares in an equal-sized carriage which holds 12, and can only make one trip per 24 hours.
Really sorry, but your market went long ago.