I think I was the one who added it. I remember it initially topped out with Interstate, and I think I added "Likes the forum" as a sort-of-joke, since at the time it seemed doubtful anyone was going to reach it any time soon (I think I saw the same phrasing on some other forum I had visited). This was the same way the next rank was added.
I am of two minds when it comes to adding something between "Interstate" and "Likes the forum". While, yes, the forum has been around long enough that we are getting more and more people reaching 10,000 posts, that is still enough of a milestone that crossing it means being around for a several-year period, so I think only a small percentage of people will end up naturally crossing that threshold no matter how long the forum is around. Also, it's kind of hard to think of something road-related that's above "Interstate" and not already used.