For example you with your 72 reputation can't downvote a post that has $100 to 0 because you don't have enough SP. Correct me if I'm wrong.
Correct. At 100% my power is worth somewhere between $3 and $4 right now so that is the most I could reduce a post.
So that means that the current system is. The higher the SP the more power you have to downvote and ruin reputations.
I could harm reputations if I down voted. I generally find almost no reason worth down voting.
You can't harm reputations if you downvote a $100 post. You can only harm reputations of people with less SP then you.
I know.
You wrote "You can only lose reputation when someone higher reputation than you down votes you." so according to what I wrote above, that's not the fact.
Hmmm... I was pretty sure I could still impact people with more SP than me if my reputation was higher.
Though I haven't put it to the test, as I don't tend to down vote people.
As I see it, there is only one thing on steem that matters. And that's Money, the more you have the more power you have. The reputation is nothing - just to throw some bone to the authors. It may also take a little bit more power to downvote more posts to get him down to -0 as @dan did to @berniesanders