I'm actually rather optimistic about the future of audax in the UK. There is a healthy number of people in their 30s and 40s who are not only active riders but willing to devote time and effort through organising, volunteering to support events or committee work. I hesitate to use the word, but some are adopting a refreshing approach to their voluntary efforts that some might view as more professional (when compared to other activities).
I suspect that group's general enthusiasm for long distance cycling and audax, and their willingness to act on it, will go a long way to welcoming and encouraging new people to look at and participate in audax over the coming years.
Of course, the emphasis might change. For example, it may be the PBP becomes less the focal point it has been to date. Particularly if national associations, such as AUK, put on events that rival PBP for challenge, structure and recognition (LEL anyone?) Or if individuals go out of their way to put on showcase events (Mille Cymru, Alba etc?)
And that's ignoring the role BPs and shorter events (including 200km) have in expanding general activity, albeit not necessarily at the longer distances (although I'd like to think that many would aspire to at least one SR).