As I understand it there are two use general cases by which old content would be upvoted.

1. The author links back to their previous posts and self promotes.
2. A user discovers an author and want to look over their old content to find gems they missed when it was new. 

I imagine the passive influx of random users would be pretty minimal and that any substantial amount of voting would come from one of these two sources. Just trying to understand what you think authors are missing out on by using Steemit so that I can thoroughly think through the problem.