I think we can agree that routes to SOU are reasonably secure because there are enough PAX with time-sensitive journeys, and the alternative - slow but direct, or quicker but changing stations in London - are unattractive.
However some routes such as Scotland to MAN are vulnerable to a more frequent train service (which will be further improved next year when major works are completed in the Manchester area).
I must point out that Flybe tried EDI to Liverpool last year. It didn't last long.