I don't quite agree. If the Feds have your key from Apple, then double encryption accomplishes something. That puts them back in the original situation, which they cry about. Even if NSA can crack it, the FBI has to call NSA (or Israel) to read it instead of just using their magic key.
Pay attention. Did you not see that Apple does not ever get your key? It does not even exist on your iOS device and cannot be extrapolated from the one-way Hash. Nor is it possible for NSA or anyone to "crack it." This is a mathematical problem having to do with the laws of very large numbers. Even the most powerful and fastest of computers would be looking at more time than there is left in eternity to try every key even in a 16 character passcode when added to the things Apple pads into the key.
PGP, although robust, is not as good because of its implementation is primarily designed for smaller file use and communications purposes, and for the validation the key is usually stored in the clear somewhere on the disk. It is slow in execution.
AES, on the other hand, is designed to handle everything including up to large monolithic disk sized gulps of data.