My only question is how our intelligence services aren't able to do this themselves.
The phone has a two-step security. First there is an intrusion detection system. If this detects an attempt to force access, it will delete all the data.
Second, the data is encrypted. So if one bypasses the intrusion detection, one still only has encrypted data (and not even apple can decrypt that without the password or PIN).
The sort of encryption used is made to take a long time for breaking. It is known to be breakable, but it is "hardened" to the extend that makes brute force decryption a time (and energy) consuming process.
Furthermore, if you have "decrypted" parts of the contents by force, you are not quite sure if they are correct. At times there are decryptions which look like sensible data, but you just hit an unfortunate "second key", that decrypted into something sensible looking, but still wrong. Thus you need to verify the decrypted data, to be correct, since more than one key can "decrypt" the data and you never know if you have the right key - and therefore the "true" decryption.
At time there is "salt" added in between the data, so that all encryptions will contain gibberish parts, even the correct one, because the salt always will ruin parts of the decryption. Only using the right key, and knowing the salted parts (to cut them out), you can be sure to have it all correct. This way even the correct decryption might look suspiciously similar to wrong decryptions.
I still think the FBI can do all the steps. But not easily and not quickly. They want the data now, and not in three years, or five.