I have worked around the western edge of the Arnhem escarpment on and off for about 20 years, including work on fauna surveys. The evidence for catastrophic mammal decline is overwhelming. As Nephrurus so succinctly pointed out, Oenpelli Pythons eat mammals, and mammals are declining, so it is not a long bow to draw to assume that as the food that they eat is no longer there, the pythons themselves are going to become less common. They have never been a common species by any measure, and as the country they live in is largely inaccessible, estimating population abundance even if they were common is going to be a difficult task. As for sticking radio transmitters on these snakes to learn more about them, that is a wonderful idea- unfortunately, I imagine the mountains of red tape one would have to carve through to undertake such a study (Northern Land Council for land access, NT parks for permit to interfere with native animals, Kakadu permit to conduct research in a national park, Animal Ethics approval to put in a transmitter, finding a vet that everyone is happy with to do it, compulsory involvement of Traditional Owners- and working out financial restitution to said TOs) even before you had found a snake makes such a study proposal close to la la land. I doubt that would happen.
Native mammals are NOT being replaced by non-native mammals, and Cane toads, which are poisoning mammals that OPs feed on (quolls and phascogales) are also feeding on those animals that quolls and phascogales fed on. The hot late-season landscape-scale burns which are hitting the stone country where these snakes occur are knocking out the food plants for rock-rats, tree rats, and rock-ringtail possums. There is less food for these snakes and all major predators- it is that simple.
Sadly, traditional owners in that area have lost a tremendous amount of traditional knowledge, especially with regards to the fauna of the stone country which is quite simply-hard work. If you have the choice between gong to the supermarket for a few kilos of beef, or going into the stone country (where it is often around 40 degrees C during the day) and climbing up and down boulders through hibiscus looking for protein- which would you choose? I only know one woman TO who I am confidant can distinguish between a Children's Python and an Oenpelli Python.
I am opposed to the illegal, unregulated trade in reptiles, to the collecting mentality that sees these marvellous components of ecosystems reduced to their monetary worth or as tools to prop up fragile egos- but in the case of the Oenpelli Python, a captive population really does need to be put in the hands of our best and brightest breeders. A genetic database held by a government authority under the watchful eye of an impartial scientific body could be used to ensure outbreeding and to cross-check the legality of captive specimens to reduce poaching.
Having seen an Oenpelli Python in the wild, draped over a Kombolgie Sandstone Boulder under an Allosyncarpia in my torchlight, I know I am one of the lucky few. I hope that same snake is still in the area where I left it untouched, and am concerned that such a snake may find its way into the hands of the highest bidder, without regards to its conservation, its cultural value or its intrinsic value as a top-order predator of one of the oldest environments in Australia.