The problem is that the conversation is totally one sided. All you ever hear are people saying "wow, this is great I've never felt better!". You don't hear the swathes of people who say "well I gave up because I was bored/tired/unmotivated" so there's never any rationality around this type of thing.
As you say, I imagine the people that did continue this for 90 days will continue to be involved in fitness. I also think that a very high percentage of those people were already involved or interested in fitness.
Getting people into fitness to buy fitness programs isn't hard, there's always a new flavour of the month. You could argue that the Spartan workout surpassed the popularity in the fitness world of Insanity after the rash of gladiator style films/programs.
Again, the thing that I'm always more arsed about is failure rates after 12 months than how great 7 people felt afterwards and these types of programs seem to have high failure rates based on the handful of surveys done independently.
You'll know yourself, fitness isn't something that you work to achieve in a 90 day program. It's how you live your life; what foods you eat, the amount of activity that you do, etc. The way that they market themselves, they put a number limit on a program that should be theoretically endless; and let's be fair here, the majority of people buying this type of thing are the type who want to get fit initially rather than those who want to reach the next plateau.
These types of programs remind me very much of the "Teach Yourself X in 24 hours" where X is Spanish or Maths or something. You can't learn Maths in 24 hours, it's just not possible to shake all of the bad habits that you have and retain knowledge of the right habits in that timescale.
The lad who is using this to get fit. Shall we have a bet what he looks like in one year, because that's the challenge here? People either drop out and go back to slobbing around or finish their 90 days and gradually become unfit again to where they need to do this regime in a few months.
I dislike any overcomplication or overmarketing of fitness and this is one of those things. All people need to do to get fit is control their calorific intake and start a very simple regime of increased activity. Insanity and P90X and the like are overkill for people wanting to get fit. They market themselves as allowing people who are already fit to get to the next plateau which I have no problem with at all.
It's like when people are addicts. You don't get clean by stopping taking the drugs for a certain amount of time, you get clean by changing your lifestyle.