What you are advocating will stop Steem's growth in its tracks

There is barely any growth (in fact over the past few months probably shrinkage). New users sign up but churn is massive. People don't stick around. Web traffic is declining, too. https://www.alexa.com/siteinfo/steemit.com

Why fucking bother post on Steemit at all if there is no incentive to be here? That's already the biggest reason people leave - they were told they could post here and be rewarded for it. And they don't get any votes

That's exactly right, because the voting is being used by people who self-enrich. The (broken) economic incentives push the system in that direction. New users who show up won't be the targets of downvotes to any great extent; they're barely visible and no one has any reason to downvote them if they are being sincere in their participation. The targets will be all the worthless cookiecutter paid-vote crap that you see on trending day after day which does nothing to attract traffic and sucks the reward pool away from the newcomers.

One downvote large enough to erase someones entire earnings for a post that she worked hard on

You are contrdicting yourself here. Taking a picture of your dinner and sharing it is not 'working hard'. If you post your dinner and people enjoy seeing it and reward you a little, that's great, but if you aren't rewarded, it is the same as Facebook and hardly the end of the world. People who do work hard to make a big contribution will occasionally be downvoted too. That will sometimes happen and is unfortunate, but they will also benefit day after day when their ability to get rewards is not destroyed by parasites using self voting, paid votes, etc. Better protecting of the reward pool from the rampant and growing self-enrichment that exists will benefit those making a real value-added contribution far more than it ever hurts them.

leave here and never return

If I had a STEEM for everyone who got downvoted and said they were leaving and will never return I'd have a lot more STEEM than I have now! It is absurd, and even more than that, self-limiting. If people do leave (which they don't) then there are more rewards left to retain the rest of the users and attract new ones.

There is no perfect system but we can make it less grossly dysfunctional than it is now.