That's a reasonable hypothesis @timcliff.
I've added the chart with average weight and number of votes on comments daily. It seems that the main driver of reduction in comment payouts is falling number of votes with weight being a secondary factor. I'll post up a chart with whale votes daily breakdown and we'll know for sure soon.
edit:
The whale vote avg weights and number of votes suggest that payouts have been decreasing due to whales decreased engagement with comments.
My speculation would be that the reason for the dramatic payout fall is an increased importance of curation bots that don't target comments, only posts.
I think you are right. More and more whales are turning over their votes to curation guild bots,which target posts. When they do that they not only don't vote on comments, but they don't even use the site any more. I would guess that engagement by whales is significantly down.
I've heard that once official curation guilds are built into the system, some people are thinking of creating one that only votes on comments :)
No... wait... the "curation guilds" were the last and ultimate savior of Steem!
At least that is what I heard the last time the founders of Steem had a clear genuine thought of their own ...like 2 mo. ago.