The more relevant measure is to know the official pass rate (the number that pass as compared to the number that take the exam). (ISC)² does not disclose the pass rate
Actually, I believe that, due to the ISO 17024 standard, they are forbidden to.
Way back in the day (before they got the 17024 standard) they did disclose pass
rates, and comparisons between people who took (different) seminars. (I recall
that there was one seminar, done by a guy who was extremely popular. However,
the numbers actually showed that, not only was his pass rate extremely low, but
that you stood a much better chance of passing doing no study at all, than if you
took his seminar.)
In terms of the adaptive format being too easy, I doubt that the "more than
double" pass rate between the early days and now means much. Once you've got
100,000 people with certification, you've automatically got a huge sales force.
(Well, assuming your certification is any good at all ...)
I can't see how reducing the number of questions from 225 to 100 maintains the complexity of the exam.
CAT exams are adaptive. They don't have to ask you 200+ questions like on a static exam. Let's look at the differences real quick.
On the static exam, each question was worth a certain number of points. There were easy questions, moderate questions, and hard questions. Additionally, you could go back and check and change your answers. Some questions and answers gave you hints to other questions and answers.
With an adaptive exam, if someone know the material, you really don't ever have to ask an easier question. You can stay at a harder level and ask fewer questions. And reducing the number of questions, reduces the chance that a prior question will provide a hint to a later question.
On an adaptive exam, the exam system decreases the difficulty after wrong answers at a higher level. That means you have to answer more questions to achieve a passing result. Answer questions right, and the difficulty goes up. Maintain a higher level of difficulty and pass with fewer questions. Answer wrong a few times, and you have to answer more questions to pass. Answer wrong enough so you can't break out of the easy questions, and the system figures out you don't know enough about that subject and skips it with a fail for the domain.
Does that makes sense?
That actually strikes a chord with me. When I took the CISSP exam, I wasn't taking it for marketing or resume purposes. I was, quite frankly, testing myself. There was this CBK on security and I wanted to know if going in cold (without studying) my knowledge and opinion was in line with the CBK. It was about boosting my own self-confidence.
When I first passed my CISSP, I didn't tell my boss or any of my coworkers. I had asked about exam reimbursement and my company basically laughed at me because they didn't think it was a real certification. In fact, when I passed, the only person I told was the Novell rep that my company was assigned to because he was the only one that knew about it beforehand. He ended up telling my boss.
It was awkward too. I would go to client meetings with my boss and he would lead off with me passing the CISSP just to rub my nose the confused look on our clients' faces (even though I was also an MCSE, MCNE, and a CCNP). But back then, the CISSP wasn't as popular as it is today. If you didn't have a cert from Cisco, Microsoft, or Novell, nobody knew what it was.
Adding to @Baechle's comments, I suspect it has to do with people being more willing to publicly share accomplishments. Before 2007, I only knew what you had for lunch if we happened to be in the same restaurant. With our kids, we just need to look on their social media accounts.