Why harder? EasyJet, BA and FlyBe serve all the same end-points as the sleeper, except Fort William. The coach from Fort William even stops at Glasgow Airport, though I grant you, it’s not possible to leave Fort William after 7pm & arrive in Central London by 9. For all the others, an early arrival in London would be possible, even Dundee via Stansted.
Correction- it actually is possible from Fort William- 1910 coach to Glasgow (2212) for an early bus to the airport or just go to the airport in the evening & sleep in the Holiday Inn/Express on site
Earlier this year I did fly up to Inverness because the sleeper doesn't run on Saturday evening and getting a daytime train is double the price and means getting up at 5:30am, but there are other examples where the sleeper is far more practical than the plane, even ignoring environmental ethics.
A couple of years ago I walked the Minigaig from Blair Atholl to Kingusse, with an overnight camp. That involved an evening train to London, board sleeper, get off at Blair Atholl aboout 6:45am, walk, camp, walk, arrive at Kingussie, kill a few hours before getting a pub meal, then board sleeper for home. All done in one weekend with no time off work.
To do the same by plane would require flying to one of the major cities on Friday evening (if there are any flights), getting a bus or train to Blair Atholl (which takes some time, and if there is a bus/train service late evening), stay overnight (because trying to do this walk after travelling on Saturday morning probably leaves the timing dangerously tight), start walk next morning, camp overnight, walk, get train from Kingussie to Inverness (if there are any running on Sunday afternoon), fly from Inverness to Gatwick (if there are any evening flights on a Sunday, and I can make it to the airport in time using train and bus, and all the connections work). Might be cheaper but a lot more stress and hassle, requires a day off work, and the many connections mean more chance of something going wrong. In addition to that, I have to buy camping cas after landing and then throw it away before leaving because you can't take compressed flammable gas canisters on flights.
Starting at Fort William is equally a pain in the arse as it is a long way from any airport, and will be impossible to arrive there and start a multi-day walk early. So yes, just because something is theoretically possible doesn't make it practical or sensible.