You are forgetting one thing: the mid 2000's was the rise of the internet dating sites - sex is a lot more available now than it was before, simply because there is much more opportunity to hook up.
Who goes to see SPs? Mostly people who for one reason or another don't have ready access to desirable females. That might be because they are older, unattractive, lack confidence, lack social skills, are married, or something like that. The other major reason would be because they want to do stuff/roleplay in ways that most civilian women would refuse.
As the internet dating scene exploded, the need for average younger men to see SPs declined so fewer would have been doing it. IMO that is what is changing the overall numbers. When those guys get older, they will likely be more open to the idea.
Frankly, I don't understand why any SP would even want to see clients under thirty, or even mid 30s, for the simple reason that those guys have ready access to tens of thousands of girls for free, so why exactly do they want/need to pay for it? And what will their attitude be?