Great questions, and I applaud your ability to approach a sensitive topic with such open curiosity.
Some arguments I'm familiar with around cancel culture are below. My goal isn't to assert them -- just enumerate some of them. People are welcome to reshape them or knock 'em down as they see fit.
- The Internet is a global namespace: now you can have people anywhere in the world outraged. What used to be a localized problem and response can now turn into a localized problem, but global response.
- The ability to go back-in-time is all too easy. That dumb comment you made as a teenager can come back years and years later to haunt you (or people close to you). Maybe you had a massive turn-around already or you were recorded without consent. Often that doesn't matter.
- It is fostering an insidious call-out culture. The focus is no longer on prevention and remediation, but on scoring points and repositioning power towards "the vanguard". Legitimate victims aren't helped to heal and offenders aren't led to improve.
- No room is given to defend yourself, particularly when mobbed upon. Even if the record is set straight later it doesn't matter, as your reputation (possibly friendships, career, and family life) is in tatters.
What I do feel worth asserting is this: no matter what the current norm is, always remember you will be judged according to later standards, right or wrong. The greater the disparity in power the more important this is regardless of role (e.g. boss, parent, etc).