most posts get to the top because of others zapping them, not the poster paying.

Why does it matter who pays the price? The only way to get on the front page is for someone to pay for the space (either the poster or other stackers). I frequently make the calculation of how much it will cost to get a certain post I want to zap onto the front page. I do this because I want other stackers to notice it. For instance, I zapped raw avocado's Gox post shortly after he posted it because I wanted to make sure other stackers wouldn't miss it. Sure, some of the calculation is my desire to zap raw avocado because he did a great job with it and I want to incentivize him to do more, but that was not my primary motivation.

I suspect SN is easier to use if one thinks about the front page as an auction for space (visibility?).

It also addresses somewhat the problem of an incentive to zap. In the past, I've felt that the main incentive to zap is to encourage posters to post more things of that kind. But it means that a zap is kind of a good will payment or value for value. But if zapping is actually about paying for space, the incentive is different: it's to bring something to other stacker's attention.