I'm uncertain about it. For other platforms they usually take this decision for monetization reasons. Here, SteemIt.com or any platform that would implement this change wouldn't directly make money from it...
It's a complicated economic subject, I wouldn't be able to rationally argue about it, but I dislike the idea of the trending getting polluted by 'promoted'...
The ratio could be always tweaked. And the monetization would be the main driver here also except everyone would benefit rather than just the company itself. We can't ask users to "please browse promoted for support" -let's be real.
For example dvery 10th spot could be reserved for top promoted posts in the trending feed. Wouldn't feel polluted. And they would be marked as promoted.