I believe that framing people for possession of child pornography is a widespread practice, and that this accounts for almost all convictions on that charge. [...] If a person were to actually want child pornography, there are simple technical measures which could create a nearly iron-clad guarantee against being caught

I think "it's easy not to get caught" is not good evidence that most people convicted of an easily-non-catchable crime are innocent. It's also easy to not leave fingerprints, and security camera footage consistently shows people not wearing gloves when stealing stuff.