Aside from the sleeper aspect, removing the Fort William part of the Highlander under normal circumstances would also reduce the Fort William (and in-between stations to Crianlarich) to Glasgow travel options by a 1/4 and a connecting coach coming in from the East on the A86 is only going to serve Roybridge, Spean Bridge and Fort William (good luck getting a full sized coach back out of Tulloch) and Corrour south has no service at all. Your also removing the one direct train to London for a geographically huge portion of Scotland, whereas Aberdeen has several day trains and flights to London airports. The West Highlands needs improved connectivity not less, removing the Ft Wm portion of the Highlander would be simply vandalism.
If capacity is an issue on the Inverness leg and the Aberdeen portion is really struggling, is there any way of getting paths for the Aberdeen portion to continue to Inverness, giving a sleeper service to 9 more stations and the later arrival time into Inverness might be a first choice for some of the tourist traffic to the Highlands.