Years ago perhaps one would conduct a poll of readers with a simple question ("competition") plus a questionnaire. offering selected prizes ( e.g .a year's free subscription).
(Then by derivation perhaps look at the subjects NOT selected by current readers as an indication of possible lost market areas?) .
P.S.
I've just spent some time looking at my combined indices of aircraft subject types covered by A.I. , A.E., F.P and AM over the last four decades or so. It is pretty well dominated by Air International ( before the change) across the board, whilst Aeroplane Monthly demonstrates an interesting geographical and temporal focus evolving over the years with some anomalies. FlyPast comes in a very low third, by international subject type, demonstrating it's particular slant.
Aeroplane Monthly launched at a claimed 55, 000 copies back in '73. IIRC when visiting FlyPast's offices in the mid 80's that they were then estimating themselves as the U.K's most popular Aviation Monthly.
Perhaps this indicates a loss in U.K. interest in historical aviation internationally - part of a more parochially orientated outlook within the emerging readership?