Why does severity need to be assessed at all if we're just going to use priority instead?
It seems that a recurrent theme in this thread is that some developers have a hard time understanding that PMs might have solid reasons to prioritize low-severity issues over high-severity issues. It's like these developers are stuck with a mindset where the forest doesn't exist and only the trees they personally have direct contact with should.be worth any consideration.
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.
The point is that severity is not ignored; it does inform the priority. Most of the time there may even be a direct correlation between severity and priority.
But other (real-world business) factors also inform the priority; while severity will never change in the absence of new information about the bug, those other factors may change frequently. It doesn't make sense for a PM to reread every single ticket and reassess each one's severity when adjusting priorities, when the developer can just determine that once and record it in the ticket from the start.
e.g. A bug might be critical severity if it wipes the entire production database, but low complexity if the fix is to delete one line of code. And maybe its priority is P1 instead of P0 because the customer said they'll remember how to avoid triggering the behavior but they really need that logo color changed asap for an important demo.
So what's the point of severity?
The point of severity is that it's an objective metric determined in isolation based on specific technical guidelines that any developer on the team can follow (such as https://www.chromium.org/developers/severity-guidelines). Whereas priority is a higher-level determination that isn't purely technical.
It's like the difference between body fat percentage and attractiveness. Any "engineer" (doctor with a DEXA scanner) can tell you your BF%, and attractiveness will typically correlate with BF%, but ultimately your "customers" (romantic partners and targets) decide how attractive you are. Not a perfect analogy (priority is still something you'd decide internally, not your customers directly), but hope that clarifies things.