You look at the statistics it's been a yearly decline since the post war years.
Speaking from memory, I think there was a gradual growth in GA up to the boom reaching its peak in the 1970s, and then a decline.
It was that boom which got various firms to get into GA (Rockwell e.g.) who then dropped it as soon as the numbers fell down.
I've been flying only 10 years but the graphs in the back of FTN suggest a 1/3 decline in new PPL issues over the past decade. What this says about the flying pilot population I haven't got a clue. There may be a connection but probably only a weak one because the vast majority of new pilots give up fast. The factors which keep long-term pilots hanging in there are probably different to those which control new PPL issues.
The CAA says there are 20k with valid medicals. What was that figure say 10 years ago?
However, I don't believe pilot population size is closely related to forum activity, which has dropped off massively over the past 2 years or so.