Enthusiasts love diversionary routes, I see many suggestions that a line is electrified for diversionary reasons, or a line should be re-opened because it'd be handy in cases of diversions.
However, the reality is that TOCs seem a lot less bothered by them - whilst passengers may prefer a slower journey on a train (rather than changing onto a bus), it's seemingly too much hassle to deal with drags/ staff knowledge/ scheduling etc.
I can see the argument for having seat reservations on the S&C (the old 37s/ mk2s had reservations, which were pretty popular). I can see the argument for powerful 185s freed up from TPE. I can see the argument for some token service from Leeds to Windermere/ Barrow (that doesn't go south to Lancaster).
But the S&C is never going to be a busy main line - the speed will always be restricted by the slow heavy freight - 100mph DMUs won't make the journey much faster than the 90mph DMUs that currently run it (and there are bound to be moans about the 185s "central" doors opening at Dent on a cold winter days...).
As well as the speed problems, there's not much population between Skipton and Carlisle - even Carlisle isn't that big a place (and there aren't many people living in the southern Borders to tap in to). Maybe some 153 shuttles from Settle to Skipton (to connect to regular 333s to Leeds) could be justified, but north of Skipton there aren't huge numbers.
Looking at NRE, it can be as fast to go Leeds - Carlisle by changing at Manchester (than sitting on a 158 on the S&C) - presumably this'll get faster with electrification of the Transpennine route. Leeds - Glasgow can already be done every couple of hours on XC (via Newcastle and Edinburgh), so if you want to improve Leeds - Glasgow links then you could always extend the bi-hourly XC services that currently terminate in Edinburgh through to Glasgow Central.
The S&C was used as an example of what other rural lines could aspire to on
one of the Dawlish threads. Looking at the stations north of Settle you have...
- Horton-in-Ribblesdale 26.0 passengers per day
- Ribblehead 28.6 passengers per day
- Dent 14.9 passengers per day
- Garsdale 20.3 passengers per day
- Kirkby Stephen 43.9 passengers per day
- Appleby 94.4 passengers per day
- Langwathby 36.5 passengers per day
- Lazonby and Kirkoswald 25.5 passengers per day
- Armathwaite 13.1 passengers per day
...not in the "Breich" / "Tees Valley Airport" / "Golf Street" league of passenger numbers, certainly, but not amazing - I'd suggest that there are bigger priorities.