I'm not aware of any reliable data for numbers of riders taking part in audax and sportives.
Sportives, with their big company backing and flash websites, are certainly more visible, but are there really many more, or are they generally any bigger, than audax events?
As regards audax fading away as participants get older, the UK arm (of the biggest long distance cycling club in the world) was founded in 1976.
That's nearly 40 years ago.
The majority of those original members must surely now be either too old and infirm to cycle long distances, or dead.
Yet lots of people are still audaxing.
If we accept audax attracts older riders, it must be the case that cyclists who wouldn't consider audaxes when they were younger do so as they get older.
So today's clean shaven young sportive riders are tomorrow's audaxing beardies.