We (as a society) have realized this with other labels that could be hurtful, and have stopped using them.
The difference is those problematic labels (such as a certain N-word with a hard-R) were created and used specifically to denigrate and devalue entire groups of people, particularly for things they can't change about themselves. "Karen" as a pejorative targets behavior which a person can absolutely change.
Why do we suddenly think it's ok to just misappropriate an actual name this way?
At least partially because it's aimed up, not down. It's mocking the oppressor rather than the oppressed.
Personally, I don't conflate actual slurs with "Karen" as a pejorative. First, you can say one out loud in public without getting looks of horror in your direction. Second, it's universally understood that "Karen" as a pejorative is shorthand for a specific type of entitled, privileged behavior, not every person named Karen whereas slurs are used as a blanket term to disparage and often unfairly assign negative traits to a group of people.
I have seen someone named Karen modify and repurpose a Mean Girls quote: "I'm not like a regular Karen; I'm a cool Karen."
First let me say I'm not sure this is really such a huge deal, and I'm also not expecting people to change because of anything I post here. I get where this came from, and how/why it's a thing. Names come and go, sometimes because of negative associations that are created for whatever reason, and perhaps it's just time for the name Karen to fade out for a while.
However, I'm not on board with your logic.
Saying that using a specific name as a pejorative is ok because there isn't a previously established social norm to discourage people from using it seems less reasonable than stopping to think about whether people are negatively affected and then adjusting your behavior and vocabulary if necessary.
Saying that a specific name is universally understood to have a negative meaning about a group of people, but that the group of people
whose name is that specific word won't be affected by (mis)use of that pejorative seems very much like an "I'm not affected, so I'm not going to care" attitude.
Finally, the fact you have already seen people who feel a need to create disclaimers about their given name to qualify what kind of person they are seems pretty ridiculous, doesn't it?
Pretty sure that we can do better without much effort. (Even though I'm pretty sure that people in general won't make that effort.)