Nirth: If someone is abused unfairly there's typically a reponse of upping that person if the large majority of people agrees that this person was treated poorly which mean the system works.
I've tested the rep system and it doesn't work like that.
If you low-rate a post, that user loses 1 rep. Uprating it to "neutral" does not return that lost rep point. That post must become high-rated at which point the user will gain 2 rep points (regain lost one and gain one for high-rated)... but once it becomes neutral, there is no way to tell that it was ever low-rated so people wouldn't high-rate it to "fix" the damage.
Also, people who abuse the system will sometimes avoid being so obvious. They will target old topics, sometimes
several years old, to reduce the chance their abuse is noticed.
In fact, it is even possible to decrease a users rep with no low-rated posts even being visible (stealth-drain).
The mechanic you descibe only mitigates some damage of the abuse and even then, only within a very limited scope (i.e. very active topics).
It takes 5 accounts to abuse the system. 6 to do so stealthily.
It takes 9-10
knowing about the abuse and co-ordinating to fix the damage.
The system is heavily biased in an abusers favour.
I could single-handedly "impose my opinion" on the entire forum using the post rating mechanics. Hearing about how there are some people on this forum who are silent when they would otherwise be arguing for [i]treating other people with respect and fairness[i] because of the abuse both saddens me and tempts me to set up such a system.. but I'm actually concerned about "becoming the monster" with such a thing in my control :P
A silly worry? Possibly. I've only downrated 2 posts (not counting downrates for system testing) in my ~1000 days here and those were both spam :)
GOG doesn't care about the abuse of the rep system, so would they care if I started abusing it for my own goals, even if they are noble (i.e. fixing abuse damage)? It really makes me wonder...
Unfortunately, GOG are too busy reorganising the game cards, making animations and breaking the site to implement something useful like an Abuse Detection System, even if a simple one could be created within an hour.
tl;dr
The post rating system is not self-correcting and is biased towards negative outcomes.