Negative curation is part of the whitepaper, and it's even quoted in this post. Steemit didn't have to add anything later, and it surely doesn't need to be justified.
Downvotes were called downvotes before they were flags, that was renamed because they were used for disagreement with opinions. Turned out flags was a bit too harsh, thus it was reverted again. If you tell history, make sure you know all of it.

Constant self upvoting is harmful to everyone else, as the pool is shared between all users. Downvotes are not harmful, as they don't take anything away, they just give it back to the pool to be distributed differently.

By what you write, you seem to be in favour of a pure PoS model. Why would we need content then? There's hundreds of coins where you get your share of the inflation based on your stake.
Proof of brain can work to a certain degree. If the community makes it so. If not, we can save a lot of overhead and just ditch the content and rewards.
That'd surely help with attracting more people, look how all the PoS coins are thriving, especially those with a high inflation! /s

(ps screw autocorrect)