I think that's a very good point... many families end up co-sleeping, whether they intended to at the beginning or not, because that's the only way they could get ANY sleep. The kids were "poor sleepers" before the co-sleeping even started, in other words.
I think your test sample will also be skewed somewhat by parents not wanting to admit that their negative/punitive approach to sleep didn't work as well as they claimed. In other words -- most kids will sleep great some nights, sleep horribly other nights. "Punitive sleep" parents will tend to emphasize the great nights, keeping the horrible nights in their minds simply as the reason they need to continue with the punitive approach. Some "AP" parents will focus more on the negative nights, because we've been told so often by the 'punitive' parents that we wouldn't be having those bad nights in the first place if we'd just shut the kid in their room, let them know who's boss, and leave them to figure it out.
I'm way overgeneralizing here, of course. But I do wonder if there is some kind of effect like that. Maybe CIO moms chat amongst themselves "oh I know dear, my DD had a regression when she was 12mo, we had to Ferberize her all over again. Just goes to show how those little monkeys will always try to manipulate you given half the chance, it's so important that we stay on top of this!" In other words, CIO 'works', you just have to do it again every so often.
Conversely, when an AP baby who was previously sleeping well starts waking frequently, we're inundated with the message that we must have "failed" in "teaching" our baby to sleep. A positive-minded parent will realize that the sleep pattern change means something developmentally big is going on, and the baby legitimately needs more comfort and night-time parenting.... not just 'manipulating' us.
Now, I do think that sometimes AP'd kids can be "poor sleepers" for a longer time than CIO kids. But I don't think that's a bad thing. It's only a bad thing if you believe that needing parenting during the night is a bad thing, or that children are out to manipulate you and for some reason WANT to ruin your night-times. And that tired, exhausted babies will wake up rather than keep sleeping, just to be manipulative or because they haven't learned how to stay asleep.
Instead, it's more demanding on the parents for a certain amount of time, while they need it. Whether to age 2 or 4 or 6. But when they're older, they're more secure because they've learned that nighttime, like daytime, is safe, and their parents are there when they need them. By meeting that need in the early years, they don't worry about it when they're older and don't need us at night as often. The difference is that we've let them come to that readiness in their own time at their own development, rather than forcing them to do it for our convenience.
That being said, I don't mean that EVERY AP'd kid is going to be a poor sleeper. Far from it. Most sleep great. It's the poor sleepers who get all the press, raised as examples by the CIO crowd as evidence that AP doesn't "work". These are the higher-needs kids, who if raised in a CIO household instead, would either be "good sleepers" but have potential security issues, or still be poor sleepers anyway and be constantly punished for it.
In general, from reading tons and tons of anecdotal reports
I think that the majority of co-sleeping, extended BFing babies, start sleeping well -- ie, extended stretches and even all night long, possibly in their own beds or own rooms -- between the age of 2 and 3. That's not too bad in the long term, when you think about it.
There are some great books about night-time AP if you want more "expert" opinions. About how the huge benefits outweigh the potential issues. And I do heartily agree that the potential issues are way dependent on the child's personality... you'll read threads here in fact from parents talking about their three kids, all parented in basically the same way, who had DRASTICALLY different sleeping patterns. Maybe the first was easy and they thought "well this is working, this is easy, what's the big deal?" and then the second is a terror. Or vice versa heh.
For my own example.... DS co-slept until about 18mo when he moved to his own room. He would still come and join us during the night most nights until he was close to 3. He ALWAYS had trouble getting to sleep. Whether nursing, walking, driving, or - I confess - holding him down, screaming and flailing, I was at my wit's end - it took forever for him to get to sleep.
He's now 11, and for the past 2 years needed melatonin most nights in order to fall asleep. He sleeps well once he's down. He's secure with no night-time problems. But he does have going to sleep issues.
Is it because of my parenting, though? No. He's ADHD, possibly Asperger's. The sleep, we've recently learned, is related to that. His brain can't calm down on its own very easily. I can't imagine how it would have terrorized and damaged him if I'd left him to CIO. It's not a matter of me 'making' him 'learn' how to sleep. He has a physiological impediment that makes it difficult for him.
DD is now 2.5yo, approaching 3. We co-slept from the beginning, at 14mo moved to a sidecar bed for most of the night. A few months ago we moved the bed across to the other side of the room. So a bit before 2.5yo. Since that time, she started sleeping amazingly well. Most nights she sleeps all night, or sometimes she'll walk over and join me during the night for a nurse back to sleep. She'll often even wake up in her bed, fuss, roll around, sit up... and settle herself right back down to sleep.
We never "taught" her that, she figured it out herself when she was ready. The only thing we did to "help", was to not *instantly* rush to her aid when she would fuss beside us at night, once she was oh... about 10mo or so? We'd give her a moment to see if it was just a passing sleepy fuss or a real need. As she got older and we observed her growing maturity, we'd wait a little longer -- waiting until she actually ASKED for us. Not when she was an infant. When she was a capable, independent toddler with good communication skills. Not before then.
Anyway, this was way longer than I intended... sorry heh.