Back in the day, I was tired of my villain Damien before the story was done with him, and I joked about resolving everything early with a random meteor.

They Had To Go. Their Planet Needed Them.

I actually can't think of a specific example of a dialogue check resulting in a boss monster simply vanishing like this.

Which is not to say that players can't talk their way into skipping things, including bosses. There's just usually an explanation, or they'll still be somewhere.

That said, there are definitely cases in which you should still be able to do something you talked your way out of, but are unable to.

An example of this is in Fallout 3 in which you can convince someone to give you plot critical information without having to do their dangerous quest. If you do this, their problem remains, and they still need someone's help, but they'll never ask you to do it if you got the information via persuasion.