Talking of Aberdeen, it is pretty much built on work traffic. The city itself is just not a particularly nice place at all, it's like a run-down and overpolluted version of Edinburgh's New Town - as a tourist there's no reason you'd not just go to Edinburgh instead. Indeed, Aberdeen is also much closer to the Central Belt than the other main destinations, making the Lowlander then a connecting train more viable.
If the work traffic is declining, I can see why the Sleeper might become less viable. Indeed, I can see a situation where CS and the Scottish Government might actually consider removing the Aberdeen portion and just working the Highlander as two half-sets, one to FW and one to Inverness. The FW could then have two seated coaches, solving the walk-up "day train" issue (add the second one at Edinburgh, or convert the then-spare lounge to a TSO).
At least Inverness is a nice place. Though I'm going there in a few weeks - I'm not however going on the Sleeper because I want to actually get there