And how would that change anything as they would just move to another sub-1000 page and continue posting shit. Keep in mind that they have to fill their signature quotas and they will just do it someplace else and posting in smaller topics won't magically make them better posters.

As I mentioned in one of the previous posts, take for example NBA thread that has more than 2k pages. Let's say that you create a new thread at the beginning of each season (which would happen if you disable signatures after 1k pages) how exactly would that affect the post quality? People would continue to write the same as before.

Since majority of spam is created due signature campaigns, its up to the managers to (at least try) control that by not counting low value posts.