But I've seen accounts get downvoted out of personal vendetta

These anecdotes will absolutely happen just as all forms of trolling and abuse will happen. Should we shut down comments because people with unvented hostility or a vendetta will sometimes post "Fuck you, cunt"? I think not. Likewise we can not dismiss or downplay the necessary role that downvotes play in peer-review. There is literally nothing else in the entire system that differentiates between quality and value add on the one hand and self-enrichment on the other. Only downvotes can do that.

But if there is only more downvoting how does that increase quality?

Nobody is suggesting only downvoting. There will still obviously be upvoting. It just won't pay to upvote non-value-adding self-enrichment (because it can easily be downvoted). Better to upvote something of actual value and take the curation rewards. The suggestion in the post to change the curation rewards back to 50-50 from 75-25 and therefore eliminate the starting point 3-1 hurdle from self-voting to curation is also excellent and I've been pushing for that since genesis+three months when it was (foolishly) changed in the first place. But that still won't work without getting a handle on the (lack of) downvoting to keep the incentives anchored on the (quality and value add of the) content.