That doesn't mean it's a good idea, though. I think it almost certainly isn't. It sends entirely the wrong message, it's a much bigger pain to implement than it's worth, it might work all too well at cutting the volume of submissions and comments (if you think a crowded HN is bad, fork HN -- the code is open source! -- put it up on your server, and see how exciting that is)... and it won't work. You really think a troll won't pay five bucks a year? I think you severely underestimate the entertainment value that a troll derives from trolling.
If, in fact, HN eventually declines to the Reddit level, the solution will be the same as ever: Some of us will head off into small invite-only groups (note that invite-only is distinct from for pay), and the rest of us will migrate to a handful of other new social news sites which will thrive for a while until they succumb to the same problem, or to an entirely new problem, after which the cycle begins anew. As someone has said in the past, it's like restaurants, or music clubs. They flare up, grow, shrink, and die out all the time.