Good reasoning and good question.
I can answer simply, all top 20 post are promoted. The important question is what's their cost in % and net $ after reward returns.
The answer is; it takes currently a budget of 115$ worth of STEEM to promote a fresh post near the top with cost of about -5%. yes, negative cost, that's a 5% profit. Meaning anything less than a 5$ downvote has no negative effect whatsoever in people promoting bad content to the top.
There are two solution to this.
- More downvotes where it matters first, (the top trendings)
- Remove all profits to vote buyers by making vote buying more easily accessible to everyone and letting people willing to pay a high premium do so.
A lot of more external money would be interested in buying votes at high premium if the top #trendings were bigger center of attention.
At @Steemium we've been collecting historical data of every bidbot bidding rounds and snapshots of top 50 trending for the last 7 months. Here are some charts that might interest you https://steemium.com/#/statistics/trending