I wonder if the decoupling of SDB with the USD around 2017-week47 influenced the ratio of comments to posts, since in a post you have the 50/50 option making it more profitable to blog than to comment? Just a thought.
Regarding STEEMIT going forward I think it's a pretty good place to blog for free (despite some rough edges), where you might actually get paid for your efforts. I've noticed some of my facebook friends have started linking articles here so that is cool, and I will likely start doing the same soon (I have a much bigger following on fb currently). Anyway that's my positive spin on it at the moment!
I think the drop in curation started when the reward ratios went from 50/50 to 70/30. The amount of curation peaked in June/July last year and it then started steadily dropping after that. People respond to incentives as they say.