You are confusing access to the computer itself with access to the data it contains.
Given physical access and "unlimited" time (i.e. no more than a million human lifetimes, say), then certainly an attacker can gain access to the device and make it do what he wants.
However, if the data on the device is securely encrypted, then physical access and (reasonable) time doesn't matter. He won't be able to get at the data.
Given physical access and "unlimited" time (i.e. no more than a million human lifetimes, say), then certainly an attacker can gain access to the device and make it do what he wants.
However, if the data on the device is securely encrypted, then physical access and (reasonable) time doesn't matter. He won't be able to get at the data.