I suppose so. But it does seem a bit crazy that that 6,000 post game is going to reward something like 13 GM Credits (1 for each 500, and another for posting on the wiki). Add that to the amount of GM credits you'll get for actually RUNNING the thing, and you should have enough credits to level your character quite a bit (two levels, maybe?). Besides, a 6,000 post game shouldn't really have a HUGE list of what happened - we don't want people to write novels.
I think if someone were to make a write-up of my Tomas Quinn adventure (which is on it's 8th page - it's fairly small), it would be only slightly longer than my Blacklily Adventure (which finished at less than 8 pages, if memory serves). I really wouldn't want the write-up of "Tomas" to say much more than "The PCs were hired by Marissa to find her son. They found out that Tomas was involved in some sort of theft, and a faction of goblins, as well as a faction of thieves, were looking for the item that Tomas stole. The PCs went to an underground lair that Tomas used as a hideout for himself and his gang of child thieves."
I mean, there'd be a bit more detail than that, but I really don't want the descriptions to go on for PAGES about details that are so minor that only the people that were in the adventure would notice. So, yeah, that 6,000 post adventure probably has a lot of encounters that might not even get written up, or barely alluded to (do we really need to talk for a while about that locked door that they eventually bashed down, for example?).
I guess what I'm getting at is, if you give 1 DM credit for each 500 posts, with no limit, those mega-adventures are REALLY going to have a lot of detail attached to them that doesn't necessarily have to be there, which will prevent other GMs from using that information readily.
But you do make a point that adventures that are really just multiple threads would spawn more credits than those one-long threads.
I think a good halfway solution would be to say something like "1 credit per 500 posts" and then, after a certain number of posts, up the requirement.
"1 credit per 500 posts, until 2,000 posts have been made. And then, 1 credit per 1,000 posts." Or something like that.