I think what you want is for someone (yourself, me, the author) to review newspapers or some similar source and determine how the frequency percent changes over

I think what you want is for someone (yourself, me, the author) to review newspapers or some similar source and determine how the frequency percent changes over time for the word "said". This is a reasonable request, but I also think it's fine for the author to state it _as an expert_ that newspapers continued using said at a similar frequency. The story they tell us plausible, and I don't really think the burden of proof is on them.