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.

1. More downvotes where it matters first, (the top trendings)
2. 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