I'm not arguing that we shouldn't upgrade the HML with reasonable upgrades like electrification, better rolling stock and so on. But, massively expensive upgrades like tilt which can't help the rest of the rail network are unlikely to ever pass economic muster. You'd go for tilt if there were a very good case for the journey time to be cut by 30 minutes or so AND you had already exhausted all other available options like speeding up Edinburgh to Perth journeys or electrification. But, the economic case for this would hinge on people doing high-value trips which are time-sensitive - your business travellers, essentially. Tourists and normal folk really just need trains to be more reliable, more frequent and have more capacity. Between Scotland and London there are plenty of business travellers who currently take the plane, and the promise of HS2 and future extensions is that train can win these people over and cause major modal shift for many thousands of important passengers a day. The numbers of people going from the Central Belt to Inverness is at least an order of magnitude lower.
If the number of passengers is that low, it can and will be more economically worthwhile to run a flight for them instead - this flight could then run even faster than any practical rail upgrade could allow. If it's a small plane, you don't need to find that many people willing to pay a premium for the flight to be financially viable. The point about electric aircraft is that they will be able to take on flights like these far sooner than they can take on longer routes, and they'll remove most of the environmental impact.
If the flight is disrupted due to poor weather, then that's just an act of god. At that point, business travellers aren't going to be that massively upset. They can still take the train - it'll just take longer. The loss from the occasional business trip being cancelled or delayed can be easily outweighed by the reduction in economic costs the rest of the time.