It's interesting to speculate on the likely trends. By then there could be only 1,200 French participants with an average age of 65. US participation could be about 200, again with an elevated age profile. I think UK participation will be a lot higher after Geraint Thomas wins the Tour, setting off a wave of interest in racing which has an Audax echo culminating in PBP 2019, much as happened with Germany after Ullrich and Denmark after Riis. The decline in participation from the traditional nations will be offset by a wave of interest from Asia, Russia and South America with a younger age profile. There will be a twin track PBP with a load of old white geezers and a rainbow wave of younger riders.
The real unknown is whether there will be enough volunteers to run the event, given the age of those on this year's.