Bob
I have always favoured the "cascade" argument. I recall from the distant past that I ran my first Three Peaks because everybody else in my club seemed to do it and I wanted to "belong". And it didn't hurt a bit.
FITC gets blamed for many things, probably with justification and often by me,: but it was published in 2004 which is a few years before 2010.
However what is undeniable is that the steepest ever increase in membership of the FRA was in the two years following publication of FITC. The annual increase then became more gradual. It seems to me that the first and more direct influence of FITC was to encourage people to go fell running and join clubs; and then the secondary factor was as above.
Perhaps you could offer every completer a free copy of FITC and you could analyse their responses from "my inspiration" to "never heard of it"