You cannot assume that the security of a composite crypotsystem is max(system1, ..., systemN), i.e. that bolting a secure system to an insecure one is at least as secure as the most secure system.

But, the scenario described - that you've responded to - isn't the max of an unordered system; it is the max of a composite system with a specific order. In other words, perceived weakest system is run always before perceived strongest system.

And, if you are saying it still doesn't matter, and if payload1->weak_crypto->strong_crypto causes a vulnerability, what keeps the same vulnerability from occurring naturally with some payload2->strong_crypto that doesn't use weak_crypto?

In other words, why would you choose strong_crypto if it has vulnerabilities with certain payloads but not other payloads? If these vulnerabilities are truly known for certain payloads, then why not inject a decision to avoid strong_crypto if the payload is known to cause vulnerabilities?

Apologies for the first-grade level questions and fervor. Maybe this is why crypto is hard. Or maybe things that are this hard to explain and understand should just not be depended on. I've experienced both.