Not every curator can make more money than a self vote.

Yes

Even in a system with 50/50, the people who vote in the end WILL get less steem than those who vote at the beginning

True but not so relevant. Under a 50/50 system the average curator makes half as a much as self voting.

This is why downvotes are needed to help close the gap further. If you self-vote in an abusive manner, yes you may get 2x compared to curating, but you may also get downvoted and earn nothing. The added risk means you may be better of curating.

That plus perhaps at least some people would rather do the right thing. Maybe they aren't willing to give up a huge profit when the difference between self-voting and curating is 4x and there is almost now downvoting (as is the case today), but perhaps they would be willing to do the right thing if the difference is only 2x and from there downvoting is a real risk. Ultimately we'll see.

Instead, more people need to think of voting as donating

Unlikely to happen, if it was going to happen it would have by now and we wouldn't be having this discussion.