<…> Here is my suggestion:
On a weekly basis, moderators can check how many posts ( non spam) a user has posted and rank them higher based on post ( no spam) count.
Ranks, as we know, have the Activity component. Even if that counter were to be solely taken into account (alongside your mod suggestion), checking every account every week would be pointless, since an account only postulates to change rank when a given Activity threshold is surpassed. Therefore, to be more efficient, the revision would have to be performed just when the account met the Activity required for his next rank (as opposed to every week; bear in mind that, currently, roughly 56k posts are created of the forum per week).
Fine, let’s continue. A moderator, who probably has a busy window to deal with reports, would also need to set time aside to go over a whole load of posts for a potential new-rank-to-be type account, and decide, as per his own and singularly unique criteria, if a postulating account ranks-up or not. If he doesn’t, then there is all the related drama that is foreseeable, coming as a tidal wave to Meta. When the account keeps posting, and reaches the next rank potentially as per his new Activity, could he actually postulate now, having being rejected on the prior rank?
It’s a lot more complicated than it seems, and it centralizes the ranking-up process way more. The Merit System does have many flaws, but it is potentially way less biased than having a single person having to decide if a given account’s rank-up. And that is not even considering the mod time factor, which is currently diluted amongst a bunch of volunteer (or such) people awarding this Merit thingy in their free time.