Before I go any further I have no hard data to prove my thoughts either way!!
Filofax themselves will not doubt have data based on their own product sales. However, they aren't the only source of organisers or planner products these days, it's a much larger market than it was say 10 years ago, but may be not as big as 40 years ago (that would be the 1980's!)
Mobile phones are mainly sold on the performance of their cameras or battery life and to a point the market has plateaued with new phones being not that much of an improvement over the previous years model.
We don't see phone manufacturers boasting about how their new gizmo will improve your productivity! Apple recently added a Journal app to their iPhone operating system, but not the iPad operating system which I think is a mistake. I've not even attempted to use it though. I much prefer my untidy writing in my paper journal with my fountain pen!
Yes I'm sure we all use digital calendars and reminder apps, I do, but I rarely have them open on the screen for very long! My A5 though is always open on my desk next to me, so I can quickly glance at it to see what I am supposed to be doing etc.
I personally think that the number of users might be increasing, not massively. Looking at the responses to questions on Facebook Groups from new member applications to join the groups, I see a lot of people stating how they used one xx years ago and they are returning to using one this year or last year and have then discovered the internet community.
Welcome to all our new Filofax and returning users.