You get same return no matter vote for what, the only factor is downvotes cast by others.

this is true.

The pure-linear approach work best in the situation where the community members are actively flagging those bad-quality posts. In a upvote-dominated system, where only a few minority doing policing job, then the flat reward-curve will just incentivize people to cast random votes.

so, linear-approach would work better if we could have a mechanism to compensate the down-voters. for e.g. maybe we could revise their voting power if a down-votes surpass some threshold (say, >50% of upvotes) so at least they wont be penalized on their voting power by doing good flagging job... it would be best if could come out with some rewarding scheme.