Yes, exactly, perfectly secure steganography would be like a philosopher's stone of perfect compression for the underlying media. If you want to send something

Yes, exactly, perfectly secure steganography would be like a philosopher's stone of perfect compression for the underlying media. If you want to send something steganographically, you just encrypt it and then "decompress" the ciphertext into a cover message. The alternative--the channel oracle concept--is a non-starter for arguing real-world security because it assumes that one can efficiently draw repeated independent samples from some arbitrary "history" of the channel, which is like saying you can pause time and take as many photographs of a particular scene as you want, each with its own random observed quantum states, before deciding which picture to post to facebook. The way around all this, of course, is to just communicate over an encrypted channel, which is exactly what Tor and other censorship circumvention systems do. But even then you have to rely on unprovable statements about the plausibility of the fact that you are using a secure channel under the given circumstances, or about the inevitable leakage from the particular implementation of the channel. This is actually exactly my current line of research: building a better framework for separating the components of real-world steganographic systems into the parts which can be proven secure, and the parts which cannot. Your last statement does a surprisingly good job of capturing why arguing the security of steganography is so hard: information-theoretically, there is no way to prove that some observation in nature has zero mutual information with some other observation. The entire field of machine learning is devoted to finding signals in massive piles of seemingly meaningless noise, and any real instantiation of a stegosystem will inevitably make some noise that could, under the right circumstances, be detectable.