So, back in the old days, "rank" ("Sr. Member", "Hero Member", etc.) used to be determined by your post count. To write that out in some symbolic form (that I'll iterate on as we go):
rank ≈ $postCount
Using the "Hero Member" rank as an example, all you had to do in order to reach that rank was make
501 posts (posts that survive moderation and don't get deleted, that is). People could
max out their rank in just a few days back then...
Spammers be like:

Then, the first major
intervention took place, and rank determination became:
rank ≈ $activity
If you back-read Meta, you'll see that people really struggled at the time to understand what this "activity" thing was.
A precise definition of "activity" isn't important for this post, what's important is to understand it conceptually: It's a way to slow down the ranking-up process. To reach "Hero Member", you now need to write 480 posts spread out over 480
days. For the purpose of increasing your rank, there's no real point in posting more than once a day; technically, there's more to it than that, and you can make just a single post in a given 14-day "window" and then make 27 (14+13) posts in the next 14-day window and have your "activity" jump up accordingly, but, that's an implementation detail and shouldn't distract you from the central idea of an effective rank-wise 1-post-per-day speed limit.
Spammers be like:

Then, the second major
intervention took place, and rank determination became:
rank ≈ lesserOf($activity, $merit)
Now things get really tricky for spammers, because even making 480 posts spread out over 480 days isn't sufficient to reach "Hero Member". You'll also need to accumulate roughly the same amount of "merit". The idea here is that some of your readership will award your substantive/appreciated contributions by marking those posts with "merit". In this "new era", you can't just rely on patience in order to rank up, you also have to try to post things that might be appreciated: If nobody finds value in what you have to say, then you'll never rank up...
Spammers be like:

We're now up-to-date, and I'd like to propose a third "intervention":
rank ≈ lesserOf($activity, $merit - $carry)
The idea here is that there's a new term called "carry" that will slowly de-rank you if you keep posting/doing things that nobody appreciates. That is, you can't just apply yourself to the task of
achieving a given rank, you also have to apply yourself to the task of
maintaining it. Just like your "merit" is a >=0 balance based on logged events, your "carry" is also a >=0 balance based on logged events. There's a more general version of this idea where different forum actions attract different amounts of "carry", but, the simple version that I think should be experimented with initially is just one where there's a single type of system-generated "carry transaction": Whenever you make a new post, your balance goes up by 0.1. In effect, you'll need to earn at least 1 merit for every 10 posts you write (on average) if you wish to prevent your account from slowly drifting toward a lower rank. And you'll need to get beyond a 0.1 merit-to-post ratio if you're aiming to increase your rank.
Spammers be like:

I think this would help with a few things (without impacting any even halfway-decent posters):
(*) It would slowly decay "airdropped" merits that either belong to accounts that have now changed hands or that were awarded in the first place to users that would have been unable to earn those merits organically. Basically, I'm thinking of accounts like
mich (that account has only managed to earn 36 merits over
thousands of posts, but it has the rank of "Legendary" and wears a paid signature). If an adjustment like what I'm proposing were put in place, then the person behind that account would have to re-think their whole approach to the forum (if they wanted to keep spamming their signature, that is).
(*) It would make it harder to buy/sell accounts. As in, sure, you could buy a "Legendary" account, but if you're going to just post mindlessly with it to make money, then eventually the "merit" that came with the account will be undone by the "carry" that accumulates each time you make a post. (And then, hopefully, the account will be considered worthless and can stop being traded.)
(*) It would make it harder to buy/sell merit. As in, sure, you could cheat by buying some, but, you'll likely have to do that
repeatedly, because, as with the previous point, if you're just going to post mindlessly, then eventually the "merit" you bought will be undone by the "carry" that accumulates each time you make a post.
(*) It would encourage more mindful posting in general, I think. A small "cost" risked by the poster each time they submit something encourages them to ask "Do I really need to be posting this? Do I really think that people might appreciate it? Or is this likely to be a post that only contributes to me losing rank?".
I was previously against this sort of thing:
Yep, I completely agree with you. I'm against any kind of de-ranking or account "erosion".
I guess, my view on this has changed over time, and I now think that the right amount of de-ranking "drift" can raise the forum's signal-to-noise ratio while only really frustrating accounts that nobody would miss. If you think about it,
rank adjustments like the one I'm proposing shouldn't affect organic usage of the forum. As in, why would someone coming to Bitcointalk so that they can ask a question or post a scam accusation, for example, be put off by the idea of a subtractive term in the rank calculation? They wouldn't care one way or the other, IMO. I think the only users that might feel something like "No. This will ruin Bitcointalk!" are the users that view the forum as some kind of weird "writing job". (And while it'll always be the case that some amount of the user base will only ever be interested in what they can
get rather than what they can
give, at least with the proposed adjustment in place it'll also be the case that their chickens will eventually come home to roost unless they also throw in a merit-worthy post once in a while.)
Two final thoughts:
(*) I can't really say that I'm happy with the name "carry". Maybe someone else will have better luck than me with finding a good name for a subtractive term in the rank calculation.
(*) I don't think "carry" (or whatever it ends up being called) should be something that's displayed independently. In my mind, it'll affect the display of your merit balance. As in, if you have 100 merit and 5 carry, then your merit balance will appear as 95. (I guess, for
display purposes, your merit balance should then be clamped to prevent it going negative. Otherwise, it'll be kind of demotivating for most new users to see their merit balance as "-1" after making their first 10 posts.)
(I haven't managed to state things exactly how I would have liked, but, my wife is shouting at me from across the house to come and eat, so, I'm going to stop fussing this post now and go get some chow.) 