You're right in that the system is vulnerable, but this doesn't mean that it's not "secure" in practical terms. One could argue that the phone is secure becaus

You're right in that the system is vulnerable, but this doesn't mean that it's not "secure" in practical terms. One could argue that the phone is secure because even Apple has no way of recovering your passcode, nor do they have a master key that might give access to your data. That's fairly secure. On the other hand, the system is insecure because it can have updates applied (which is what the FBI wants Apple to do) without requiring user consent, only physical access. Imagine that updates required a user to put in their passcode, or otherwise wiped all the encrypted data; it would be more secure, but is that enough? Theoretically, there are still ways of getting at the data - someone might find a bug in the software, or they might physically crack open the CPU to get at a piece of needed data. Security is always a balance - we want to trust Apple as little as possible, but I know of no way to create an invulnerable system.