The Caledonian Sleeper Franchise runs till 2030 - how does that fit with a concession model and is it likely to be the last franchise standing in GBR land?
I think we may see the CS go, its the most heavily subsidised franchise in the UK with not much reason to keep it IMO especially post HS2. The rolling stock would be better used at Night Riviera which though it requires subsidy I think it requires a lot less.
ScotRail and Caledonian Sleeper are devolved to the Scottish Government, so that will be for them to decide. Not GBR or the DfT, unless the UK government decide to claw back those powers (which will never happen in the current climate).
And they definitely will not be thinking about what's best for the Cornish Sleeper.
As for the HS2 argument, shaving 40 minutes off the journey to Glasgow and Edinburgh might kill off the Lowlander if berths can't be filled thereafter, but it will definitely not kill off the Highlander which doesn't serve Glasgow or Edinburgh at all from the south (unless you want to clutch at the straws for Dalmuir!).
GBR is very much a fancy marketing ploy for something that is predominantly a model to overhaul English operators, with the exception of infrastructure and ticketing which will apply UK wide should the current plans make it to reality.