Contrary to popular belief, stating that something belongs in the public domain does not actually put it there. Instead it is effectively just a bare license, without what lawyers call "an interest". So it may be revoked at any time by the author, or in the event of the author's death by the legal heir.

So the user doesn't actually get any meaningful legal protection. That's bad. But it gets worse. Releasing your code and calling it public domain means that you have the possibility of being sued if it doesn't work like someone else thought it would.

It is much, much better to use a simple, permissive license instead of trying to make it public domain. It protects your users better (you are less able to revoke it), and protects you better (you get some protection against lawsuits).

As for copyrights expiring, the simple rule for that which is likely work for all of our lifetimes is, "Is this text older than Mickey Mouse?" Seriously, every time Disney's famous movie Steamboat Willie threatens to become public domain, the law is changed to extend copyright.

But there is another way for something to become public domain. By law, work created by the government is not copyrightable.

(Note. I am not a lawyer. This is not legal advice. But I have spent time with people who have a serious interest in the law.)

Update: JavaFan is correct. I am describing US law here. Other countries vary widely. Though the Mickey Mouse rule is as close to universally true as Disney's lobbying can make it.