It was obviously a trade-off. n^2 is less prone to abuse, but has issues with users feeling like their voice doesn't matter. linear gives users more of a voice, but leaves more of a door open for abuse. The developers (and witnesses) chose to favor the latter.
As far as the abuse-mitigation, the WP lists downvoting as the protocol's solution to the abuse. The community/stake-holders are basically going to need to decide if it is something they want to prevent/fight/downvote, or if it is something that is basically going to become socially acceptable, and just accept that there is going to be a share of people in the community that do it.
Curation rewards are a separate discussion. It is a valid one to have, and there are good arguments to be made on both sides of the discussion. There is part of me that would like to see them entirely disappear, but then there is a different part that would like to see them raised to 50 or even 75%. The reason has nothing to do with the abuse though. Unless it were taken all the way to 75%, most stakeholders who are interested in using the platform as a tool to increase their own personal stake (not trying to say that in a bad way) are still going to be able to do it more easily by voting for themselves and getting the author+curation reward than trying to do a good enough job on curation to get curation only to surpass what they would have received by just self-voting and taking both rewards.
Curation rewards seems to be the root of many of the problems here. At the end of the day, it seems like Steemit has too much content for too few readers. We need to do whatever will reverse this equation, and let it then sort itself out.
Curation rewards really only benefit the users who already have a lot of SP though. Even with thousands of users 'curating', increasing the amount paid for curation rewards would not do any good unless they all already had the SP for their votes to be worth something.
I see your point. We want what is best for the site, but how to achieve it... Just speaking anecdotally, I see users try Steemit for a bit then leave. I dont know what part a lack of rewards played in the lack of stickiness.... I see that shear volume of curation isnt the total answer.