I wonder if the dropout rate increased because people who weren't sure they would do it had to enter or lose their chance.
A lot of marketing is based on 'Your Last Chance'. This seems to be amplified in Audax because it's an activity defined by deadlines. There are a lot of other time-dependent factors in play, the cost of transport and accommodation vary with demand.
Qualification is also a mind-game with yourself. For €30 you can transform an SR series into a high-stakes game, amplifying the motivational factor. If you wait until after qualifying to price up PBP, it may be beyond your budget, but the €30 has been good value. It's a small price to pay for a dream that increases your fitness.
There's a large contrast with my PBPs in 2007 and before. First you qualified, then you entered. Accommodation was a cheap campsite close to the start. Along the way there was never any thought of anything being sold out. That culture persists in Semaine Federale.
The kind of planning that participants from outside Europe were first subject to has been imposed on those based closer to the event. It has weeded out those with a more casual approach, so there'll be 1,000 less 'happy go lucky' riders. They'll just go to Semaine Federale, and spend more money there.