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Downvotes aren't strictly necessary.

Curation of content, including nuking filth and spam, is strictly necessary.

Then I suppose the real question is: who determines that curation?
In the case of Hive, stake makes that determination
In the case of Steem, a corporate identity is responsible
For Blurt, the regents and a council

They all have "down votes" (demonetization).
The designer, Dan Larimer, mostly spoke for Hive's model so I won't rehash all the morality of why stake should make the determination.
Blurt seems basically the same as the YouTube model when it comes to demonetization (not at all the same when it comes to censorship).
Steem has YouTube's demonetization AND censorship model.
Last we heard your thoughts on the matter, the morality of who makes this determination was more about "money for enemies". That seems to not be the current direction, so it'd be interesting to hear your current thoughts on the matter