That's a very good shout to be fair I hadn't considered that
The Highlander is not Mk5s at the moment, though. It’ll still be Mk2s/Mk3s until September at least, I think. I think when the WHL is closed at the end of this month/start of next month, the plan is to divert the Fort William portion to Kingussie (leaving it attached to the Inverness portion north of Edinburgh, and running it to Inverness for servicing along with that portion’s regular 8 coaches) from where passengers can be taken to Tulloch, Roy Bridge, Spean Bridge and Fort William at a sociable time. This isn’t necessarily ideal, as stations between Corrour and Glasgow are left without a sleeper service or any replacement transport (Corrour is impossible to do anything about, and admittedly it won’t be much of an inconvenience for folk going to/from Dalmuir, Dumbarton and Helensburgh to use the Lowlander, but for places north of Helensburgh it might become needlessly awkward). However, this is what CS have decided to do on this occasion. Perhaps there were good reasons why diversion to Oban was deemed impossible/not practical on this occasion? Passengers (and the Fort William based crew, whose shifts are long enough as it is) will reach Fort William much quicker using the Kingussie diversion than they would by going to Oban and being bussed from there, which may be a consideration.
While it may be an interesting route for enthusiasts and casual leisure travellers, and ensures that the Loch Lomond villages, Crianlarich and Tyndrum are still served, I’m not myself convinced of the usefulness of the Oban diversions. While Oban may be a convenient railhead for North Argyll, Mull and the southern Hebrides, it may be somewhat out of the way for a lot of the Fort William-bound passengers (whose likely destinations are somewhat further north, in Lochaber or the Great Glen) and the poor Fort William crew who need to get home. I for one (as my actual destination is a wee place 15 miles east of Kyle of Lochalsh, which is most easily accessed by a bus from Spean Bridge through Invergarry and Glen Shiel which connects well with the sleeper) would not find a diversion to Oban useful, and would make sure that I was booked on the Inverness portion if that happened (a bus from Kingussie to Spean Bridge would probably be more helpful than an Oban diversion in my case, though I would probably be better off just going to Inverness and getting the Kyle train).
