We can actually implement "merit points", a fine category in which people got promoted by being "useful"
Users of the forum read the replies and when something is good or very good, the people will get a reward, with sufficient rewards the person is considered of a higher rank, not because of how many post he has done, but because how many likes he/she accumulated, which means people are really liking what the person is writing.
Furthermore you can remove the post count functionality and only implement the merit ranks, which I believe is the way to go. Because, yes, a higher ranking person means he/she received more merit for clear answers and he can be trusted.
Either way, the merits wont affect the normal forum functioning and the users can totally ignore ranks if they like, there is no penalization whatsoever for people who don't get likes.
It's just a tool that can be implemented to improve the quality of interactions.