Cosby was convicted of a sexual assault that ocurred about 14 years ago (not 30), which is why it was still within the statute of limitations. And a jury decided the outcome based on all the evidence. I think he got a fair deal.
Because a crime ocurred 30 years ago does not make it any less of a crime, it may just be harder to prove or substantiate. But there are many reasons someone does not report or reveal what took place much sooner.
The situation with Brett Kavanaugh, the nominee for the U.S. Supreme Court, did occur over 30 years ago, but the accusation made against him does reflect on his character, and honesty, and it does seem appropriate
to reveal, given the position he is seeking. I don't think this is a case of
"let's go get him"--his accuser seems quite credible.
I'm not sure I see the same "can of worms" that you do. I think more men and women are suddenly feeling empowered to come forward and reveal past sexual assaults, and I think more people are listening to them and taking them seriously. But, those they accuse, can sue them for slander, or libel, or defamation of character, if they feel these assertions are false.