Given that Scotland to SW (pre Covid) was circa 85,000 and mid 90's it was about 185,000, the sleepers probably carried 100,000 a year.
Over a 7 day a week service, at best that's 140 passengers each way on average, and so hardly the sorts of numbers which make for a viable service.
What may change in the future is pairing if HS2 service with the sleeper to/from Cornwall.
Currently the last service from Edinburgh to Penzance (not using the sleeper) is at 13:30 and arrives at 00:35. With HS2 this shifts to being able to leave Edinburgh between 15:00 and 16:00 and changing at Old Oak Common (3 hour journey time), this makes taking a half day more viable and so being able to get to Cornwall to then go to sleep in an actual bed.
Using the sleeper you can leave Edinburgh at 17:30 and arrive in Penzance at 7:54. With HS2 services taking 3 hours to Old Oak Common (change to get to Paddington) with the sleeper boarding from 22:30 it should be possible to leave Edinburgh between 18:30 and 19:30 (depending on the timings) and still catch the sleeper, which would make it possible for more to do a full days work and still arrive in Cornwall the following morning.
As such it reduces the viability of a SW/Scotland sleeper further.
Especially given that according to the timetables posted here
https://www.railforums.co.uk/thread...n-to-their-current-scope.215137/#post-5038938
The old sleeper left Penzance at 1650 and arrived in Edinburgh at 8:15, assuming similar timings for the southbound journey not too different from using HS2 and the sleeper.
Whilst going the other way would be slower, as the sleeper arrives a little after 5:00, so even the earliest HS2 service (assuming a 5:30 to 6:30 departure time) would be arriving 8:30 to 9:30, so at worst 45 minutes later than the old sleeper but with a much later departure from Penzance (21:45).