Insider threat is always been the primary threat. Also, you will never stop a truly motivated attacker. There are no systems that exist that cannot be broken into with enough effort. The only purpose of encryption is to "protect" the data long enough that if the data is cracked it is no longer useful. The weak link in the chain is usually some idiot with too much clearance that found a USB drive (or something) in the parking lot and plugged it into a "secure" system. Even air-gaped systems are not safe.