You don't understand OpenID. As a site, you don't have an OpenID provider; your users do. So if their provider closes, or their site is down, that user can't log in, just like if that user's email server is down, he can't check his email.
That said, you shouldn't bottleneck the login/registration process through OpenID or ClickPass. Make sure they can login to the site, through the site, or through OID or CP just like Hacker News does.
