Just some of my thoughts.
I think immutable characteristics are largely considered bad places to make attacks and a lot about appearance qualifies for that.
Neckbeard as an insult seems to me like more of a reference to someone being unkempt or referencing some inactive lifestyle. If you think about it like that it's an attribute that is within someones control and speaks in some part to their character.
Obviously "a symbol of moral decay" is pretty extreme (feels very Petersonian), but as a generic insult it doesn't feel out of line.
I checked some of your posts on this and it looks like you mention having friends you could describe as being 'neckbeards' and I'm curious if the 'unkempt' / lifestyle connotation for a neckbeard is there.
If you think this description for the word fits and it describes your friends then your point reads like 'someone can be unkempt and a good person' which I think most if not all people can agree with or at least understand. If you think that doesn't describe your friends but that the word is meant to reference someone character in some way, then I think using the word for them confuses the meaning. If you think the word is meant solely to mean "physical attribute (derogatory)" then that probably just comes down to a difference in experience.
People get called neckbeards most of the time for their behavior. Someone with neck hair who is well put together wouldn't get called neckbeard in my experience.