PLaying devil's advocate here but…
"Carron Valley was a red (mostly easy) but with one superb feature called Kelpies Staircase. Kelpies was the reason that CV was a red – a nice rocky steep bit."
"Only did a few laps of CV once but that staircase was the one 'challenging' section."
If there's only one feature that made the route a red, or which is significantly above the difficulty of the rest of the trail, it makes sense to squash it. Having one harder part in an otherwise easy trail is generally a pretty bad idea, it doesn't even need to be hard to catch people out once they've mentally reset the difficulty level to "easy". I guess this could be with a mind towards regrading the whole trail, or could equally just be about levelling the difficulty across the trail.
Having said that, the best solution to that's to signpost it and if need be chicken run it, not to demolish the whole thing 🙁 I looked at a video justnow and it looks like there was an unofficial chicken run formed down the left hand side already, FC absolutely hate that, and tend to treat it as riders voting with their tyres and saying "We don't like this section"… But it shouldn't have to be all or nothing. It doesn't even make much sense since it'd be less work to build a clear, well surfaced chicken run and bash in a couple of posts.