I too struggle to see how all species of reptiles and amphibians (bar one) are believed to be experiencing a decline in abundance. Longqi, you stated they are all dropping exponentially (well, you never stated to what degree so you must mean exponentially) and are therefore considered not to be stable. Based on this, ALL species are heading for extinction?... I doubt it.
Yes, more accessible species will be collected faster, and easier, but many breed young and multiclutch and will therefore disperse amongst the market quicker, lowering the appeal for continual collection. Species with high colour and pattern variation for instance, such as Nephrurus spp., will most likely be sought to supplement existing genetics that support new morphs, however, with such a healthy captive population, what authority would grant a take permit? None with half a brain, thats who. Then again, you might say, they'll just grab them anyway and say they bred them... how is that any different to what can occur now?
Take widespread, common and (as eipper has said, Least concern) species for instance, some are kept in captivity (bearded dragons, knob-tailed geckos, blue-tongued skinks etc) however, the vast majority are not, as stated by GeckPhotographer. Collection of a limited number of animals, for the establishment of the species in captivity has proven to be quite successful, look at Morelia carinata, Nephrurus wheeleri, N.l.occidentalis and N.l.pilbarensis, well, obviously all captive species. Collectors would need to remain vigilant of the genetic availability within several seasons based on the numbers and origin of wild-caught animals. To me, collection (along with stringent regulations) will not have a negative effect on wild populations that are considered secure without forseeable threat. I base this on the mortality of reptiles on roads and by feral predators; and the lack of MVP studies into many of the widespread, commonly encountered species that many breeders are keen to acquire.
Take species such as Morelia carinata, they were collected and they are quite common in captivity now (to the point where they dropped $10,000+ in value in a matter of years). However, following carefully regulated taking, proper insurance of genetic availability (although, can we really rule out that they aren't being inbred now?) and knowledgeable keepers, where have all the reports gone stating that this species has now sufferred a noticeable drop in wild population numbers since original collection by Weigel etc? What's that? They haven't dropped? Even after being collected? Even if they are remote species from a rather small range?
What can we extrapolate from this example to other widespread species?
Collecting species, making them available to hobbyists and lowering ridiculous price tags, to me, has the potential to cut poaching and illegal collection considerably.
If people disagree with me, prove me wrong. Provide scientific articles (refereed and supported) that disproves what myself, eipper, GeckPhotographer etc believe.