I mean I think most HC's are fired for "failing". I suppose there might be a few that failed for like being alcoholics or something, but that's fairly rare.
Like even Andy Reid got fired for "failing". He failed to make the playoffs his last two years in Philly, and hadn't won a playoff game in 4 years. It's not like colossal failure, but its failure, based on whatever the expectations were.
Coordinators and assistants routinely get fired no matter how good or bad they were. They're pretty much tied to the person that hired them. If Harbaugh gets fired, pretty good chance Roman and MacDonald are getting fired too, regardless of how good or bad they've been.
But I agree there's really no such thing as hiring a HC, coordinator, or assistant, who hasn't been fired for some sort of "failure" in their past. Otherwise you're just hiring somebody with basically no coaching experience at all. Basically just hire like Matt Birk to coach the team.