A backdoor does not necessarily have to be in the form of a door, a wall made from cardboard will do just fine as a backdoor.
Putting a complex piece of software that you cannot disable, that is potentially reachable from the network, and that won't be updated into every machine is the software-equivalent of building a safe with a cardboard wall: Even if it's not intended as a backdoor, it still has to be treated as a de-facto backdoor for security planning purposes, and whoever uses cardboard to construct a safe wall is to blame.
Also, you do not judge security based on what has already happened nor on what you can prove to be insecure. The default assumption is that things are insecure, unless you can demonstrate that there are good reasons to believe that it's not--just as everywhere else in reliability engineering. A bridge is not assumed to be safe to use until it collapses or is demonstrated to be unsafe--a bridge is assumed to be unsafe to use until it is demonstrated that to the best of our current understanding of how to build reliable bridges, there is no reason to expect failure. A bridge where the builder makes a secret out of how the bridge was built is never considered safe.
Putting a complex piece of software that you cannot disable, that is potentially reachable from the network, and that won't be updated into every machine is the software-equivalent of building a safe with a cardboard wall: Even if it's not intended as a backdoor, it still has to be treated as a de-facto backdoor for security planning purposes, and whoever uses cardboard to construct a safe wall is to blame.
Also, you do not judge security based on what has already happened nor on what you can prove to be insecure. The default assumption is that things are insecure, unless you can demonstrate that there are good reasons to believe that it's not--just as everywhere else in reliability engineering. A bridge is not assumed to be safe to use until it collapses or is demonstrated to be unsafe--a bridge is assumed to be unsafe to use until it is demonstrated that to the best of our current understanding of how to build reliable bridges, there is no reason to expect failure. A bridge where the builder makes a secret out of how the bridge was built is never considered safe.