how many people need to be offended by "eastern" for it to be removed?
Zero. It's not about offense, it's about harm. Once more marginalized people are harmed by a word's misuse than are served by its appropriate use, the word needs to be retired.
This isn't an exact measure, of course. But it's also illustrative of why the pearl-clutchers are missing the point when they say "why can't they just have thick skin?"—because it's not about offense, it's about harm.
No one is being harmed by the word "boomer," and boomers as a class aren't marginalized. They're just being annoyed that younger people are using it to call them out on the ways they're not being good citizens.
No. It was words that dehumanized people, which justified putting them in camps.
Yes. It says "you can't trust me to have your best interests at heart, because I don't see you as a person."
Except that the word's normalization sends a message to the racists that it's safe to continue dehumanizing that group. That's the value a racist gets out of it; not "comedy" or "free speech," but "this is a safe place for your racism to fester in the open." It allows for groups of them to come together more easily, which gives them even more power. It's in the best interest of an inclusive society for that comfort to be denied them.
But you're the only one who can't decide what you're harmed by.
Slurs are all of those things, though. They're an attempt to tie evil things to a group of marginalized people, and thus dehumanize them. That's slander (misinformation, lies, and deceit, weaponized for harm).