I don't know whether the Stagecoach X51 is taking any more passengers from Livingston to Glasgow than the old First X15 used to, but good luck to them for at least trying.
It does seem to be picking up passengers, the benefit of course is if your traveling from Glasgow it takes you straight to Shops and many work places. The old X15 went a different way and then they also started using strange stock on it.
However they couldn't make a go of the 23 from St Andrews to Stirling - that felt like a route that could have had an experimental Glasgow extension (as a way of Stagecoach dipping a toe into the Stirling - Glasgow market, plus through services from Dollar etc - could have been a nice tourist route for people at the Glasgow end.
Stagecoach had real troubles operating this route, which included running a bus empty 36 miles every morning just to start one journey, which was never that great to begin with. On the Fife side, there was more frequent services paid for by the council, so do you wait for 23 or jump on another route? The route wasn't operated by coaches ( it did for a spell but its believed the locals did not like them hence there were taken off the route) So extending them to Glasgow would have resulted in single deckers being used, it should be noted, IF that was the case then Glasgow depot would have had to operate at least one of the duties, it might have helped cut down on the dead running but X24 does a perfect end to end service anyways. Would Stagecoach want to compete and undermine the M8/909 Citylink route? The 23 was never that busy. At least with First X53 it will pick up more local passengers especially since Mackies Ma2/3 are now gone.
I think that there might be enough of a market for a kind of Edinburgh - Hamilton - East Kilbride - Paisley - Glasgow Airport service, given the lack of rail links between those places and the capital, plus the way that coaches can cater for the kind of "time rich" punters who value a direct Airport link over the option to change vehicles at Buchanan Bus Station.
Remember the spat between First and Arriva with 600 and 601? To be fair in Lanarkshire there is alot of missing buses routes but there probably is demand once covid is over to see about restore a link of some sort. Trouble was you it was quicker at times to just use Glasgow express IE going give City centre.
I wonder why First doesn't operate more buses into Maxi centre to provide a interchange for users, That would be the best option to test the water for Direct service to Edinburgh. Again with the Shotts line been improved does give most people with a short drive quick trains into Edinburgh.
Something like the aforementioned 913 looks a nice idea on paper - if you can fill it full of tourists at Edinburgh then it doesn't matter so much if it's running late en route (compared to a route that would have a lot of passengers waiting at intermediate places).
The thing is, it was used by locals else where outside Edinburgh, however the coaches that kept on breaking down did not help matters.