The history of the usage is a bit blurry to me and I expect to many others. To assign the behaviour to a particular name is rather damaging. It is worse than having your first name sneered at for being common, because Karenesque behaviour is hateful and bullying, and the moniker describes an attitude not social origins.
However, there are some truths which need to be examined and as a society we need to decide if we want to put up with. For a few decades, the way the service industry has worked in relation to large chains, is that almost any loud and vociferous complaint will result in a compensation of some kind: a discount, a freebie, an upgrade. It is too achievable, and people can let rip, disregarding the feelings of the actual individual who is representing the company. There is a way of standing your ground and being reasonable and persuasive. And not pushing the point. It's less and less possible to get to see a manager in shops etc. The ruthless efficiency of the computerised system is the boss.
The other thing is that in big companies members of staff are trained to respond to comments and questions in a particular way. There is a script and a list of options and a system that they can't over ride, they don't have the authority or training to do so, do they just have to stand there and suck up the abuse, while somebody verbose and sure-of- their ground let's rip. This just is the modern world. Personal service is not what it was because everything is computerised with a standard response and options. Training is to pick an option not to get into a debate on ethics. The benefits are that these systems are often effective in getting us all relatively inexpensive, clean, standardized products.
Also within these big companies, juniors will often rise through the tires because they are hardworking and very intelligent and they see self- promoting behaviour as ugly when it is ugly. Glitches in big systems and companies are common and will always be from now on. We will just have to put up with it as we do computers going wrong all the time, buffering and updating, and it being very hard to know how to overide these mechanical quirks.
Reviews are another thing. Online reviews affect companies. Perhaps, increasingly, disgruntled people may stop taking out their ire on the personnel and just leave a small star review.
Anyway, it's always good to listen to the point of view, even if you don't agree with it, and if some people say that others have been entitled, then I'm sure it's worth considering the view..as long as the counter argument is going to be listened to also.
People get called Karen, I think, when it is clear that they would not use their powerful and articulate voice to protect and advocate for the oppressed - only for themselves. Or that is how some people think it was and should be, but I reckon the insult is being abused now to take in anyone who the name-caller doesn't like.