Yes, if you don't give me a reason to vote for content I consider good rather than my own, I'll just vote my own

Now you can't give me 100% curation or there's no reason for anyone to create good content. So that's why we need the other measures. I need not just more incentive to vote others, but a deterrence to vote myself (or sell my votes). That's where some free downvotes come in.

A little bit of converging linear curve brings all profitable spam into the light to get downvoted.

Every measure helps but have diminishing returns and increasing costs. That's why a combination of measures is best, it provides the maximum utility, assuming we get the numbers in the right ballpark