Why set a severity if you're not going to use it? A crash is still a crash if you don't set the severity and just write that it's a crash in the bug description.
Your question misses the point entirely.
The point is that severity is an element that's used to classify an issue with regards to priority. Severity does not exist in a vacuum, and priority is given depending on context. Severity is an attribute, other attributes, that's used to determine the priority.
That's the point, everyone does not understand the business context. Nor are they expected to. That's the job of the PM it's his main responsibility, and it's the reason PMs are hired to manage teams of developers.
The point of developers is to make software that people use. So if we want to do our jobs well, we have to understand how we are creating value. Product managers may manage that information flow, and they may get the final say in decisions. But if they are specifying software in enough detail that developers can be 100% ignorant, then developers can (and should!) be automated out of that work.