I personally think there are more in captivity but thats my opinion. As far as them only being captivity in the future. Theres always the possibility for that as for all reptile species, look at the yangtze giant softshell turtle for example. At one point they were abundant, throughout the years there are fewer and fewer due to the food market and the pet trade and habitat destruction, now there are only 4 known to exist. One in the wild and three at zoos. So to answer your question, I think its a possibility for ball pythons to only be in captivity at some point in the future. Again this is just my opinion on the subject.