That option is only available in the entire forum configurations.  It is not available to individual users.  We have tried that option in the past.  It was my idea to try it.  I thought it would be a good idea.  I was WRONG.   It is a good idea on low traffic forums.  But it was a catastrophe on NAT.  We have an very high volume of post.  During the busy times of the day.  Post were jumping around everywhere and you could not find them.  Someone could start a new thread and in a couple of minutes it would be gone from the first page because there had been new replies to old post.  It became very apparent that we did not want post jumping back to the top every time there was a new reply.

We are open to new ideas.  But we have been running this forum for a long time now.  We have tried new things.  Some were good and some were not.  This particular feature was not a good idea on this forum.