I'm not disputing that it's fair to investigate that. What I'm asking is if it's fair to then call it a vulnerability without establishing that the thing is, in fact, vulnerable as a result.
I would say it's like calling the battery a fire hazard if the vents don't work, but actually that's not analogous because the necessity for vents doesn't merely arise from the need to protect against bad design of the protection circuitry. They're needed for safety even if your circuitry design is flawless. So the analogy is actually kind of poor in that regard.
This is why a distinction is often drawn between vulnerabilities and exploits — many more things can be weaknesses in a system that can only be exploited in combination with other vulnerabilities.
An obvious example is web browsers, where a vulnerability can easily be uninteresting because it lives in a sandboxed process… until you find a sandbox escape, then it is critical.
As long as you suspect there may be other vulnerabilities in the other layers, it is worthwhile investigating and fixing them, because defence in depth only works until someone manages to put together a full chain.