Don't think you should be able to get permanent mini-ranks from posting. That would devalue mini-ranks in general and a lot more people would have them. No support.
Plus I really don't see why people wouldn't stop posting after they got the mini-rank. What if they even stopped and became inactive, then a lot of inactive accounts would have mini-ranks.
The less people with mini-ranks and the harder they are to obtain, the better. In my opinion. And this is coming from someone who doesn't have a mini-rank.
I personally think it would be really annoying to see lots of people with mini-ranks, and I can easily see this happening if this suggestion was added to the site. Honestly I think if this was carried through in the far future it would just eventually end up making activity decrease because if everyone has mini-ranks, they aren't special anymore and suddenly people stop trying to get them. Seeing mini-ranks on inactive accounts of people who would get the 1k postcount then quit a month later would devalue them completely.