That's a good question. The near-certain forthcoming Scottish independence would likely kill it, I reckon, as there wouldn't be MPs travelling back and forth. And HS2 will likely kill the Lowlander. It might be viable to have a simpler operation a bit more like the Cornish one, perhaps just Inverness and FW (or even Oban instead)? If primarily for leisure travellers it could run a bit later to allow an 05something arrival at Edinburgh/Glasgow for those who do need to get there then.
Why would independence kill demand for the sleeper, just out of interest? After all, there are sleeper services in continental Europe which cross international boundaries, and enough folk (in normal times) seem to want to use the Eurostar from London to Paris/Brussels for all sorts of business and leisure travel. I am fairly sure that whatever happens in Scotland over the next few years (which is by no means a dead certain, as a week is a long time in politics) there will still be considerable demand for cross-border travel, both essential (weddings and funerals will continue, thousands will still have family over the border, for instance) and discretionary. Scottish MPs sitting in Westminster, as has been pointed out above, are hardly a reason for keeping the sleeper going. They very rarely use it.
That said, the sleeper is not the only way of travelling to Scotland. I do worry about the sleeper’s long-term future for a number of reasons. First, in the event of independence, where would funding for the service come from, given that it is not a profitable operation? Second, regardless of independence, I continue to have my doubts as to whether the business model is sustainable in the long term- I worry that the fares are prohibitively expensive for many prospective leisure passengers (who are of course not on expenses), and that (however optimistic we are) the target market of well heeled foreign tourists is unlikely to return in large numbers for another 18 months yet. We also have to bear in mind that the facilities which attract these well heeled travellers- specifically the club car and catering offering- are unlikely to be available for some time, perhaps not even until 2022. I do think that CS need to think hard about ways to fill the trains in the summer of 2021 and beyond, as otherwise the service is going to look like a very expensive white elephant. That said, reliability and customer service have improved massively since the start of the year, and those who travelled on the sleeper during the period of Covid restrictions are (ironically) far more likely to sing its praises than those who experienced it before the pandemic hit.
Another thing- I’ve never been convinced by the idea of rerouting the West Highland sleeper to Oban. Why would Oban be preferable to Fort William? Fort William is probably, in my view, a greater tourist draw given its proximity to Ben Nevis and Glencoe, various attractions along the Great Glen and of course the Road to the Isles with the Jacobite steam train and Glenfinnan viaduct. There is also some potential business, at least, in the fact that hillwalkers, climbers and deer stalkers have a direct train to Rannoch and Corrour. Fort William is also much closer to Skye, a huge tourist destination, although most Skye bound passengers are probably just as well served by the Inverness train with its connection with the Kyle line (and for those hiring cars, it’s just as quick to drive from Inverness to the Skye bridge as from Fort William).
On balance, though, Fort William still strikes me as the more useful destination- and in the tourist season is evidently a very popular one. CS did once provide a bus link from Crianlarich to Oban (at the relatively sociable time of 07:45) for those who wanted to travel to the west coast port, although I’m not sure how well used this was.