Nobody is being run out of a project over the slightest misstep; the first response to most issues will be communication, not bans. Bans from a project are the last resort, not the first. People who work in projects with codes of conduct don't go around afraid they're going to be booted.

Beyond that: what work do you need to do that you can't possibly accomplish without hurting people? It's really not hard.

In that case "be excellent to each other" should be more than enough.

"Explicit guidelines have demonstrated success in other projects and other areas of the kernel."

Many people do not start out sharing a common understanding on this topic. This HN thread is more than enough evidence for that. Hence writing down a framework to give people that common understanding.