Eventually, the requirement that Ripple users fund their new accounts through XRP purchase on the open market is the ONLY WAY for XRP to serve in the anti-spam role.
This is exactly what will happen once 50 billion XRP have been given away.
The only real difference between your PoW approach and our giveaway is that yours requires people to waste money and resources on needless PoW computations and ours doesn't. And ours allows giveaways to be used to promote the system and drive adoption and yours doesn't at least, not as much. Your scheme also allows people who put effort into building optimal PoW setups to get larger shares of XRP than ordinary people who don't. Your scheme is easier to implement. As I said, using PoW as a giveaway mechanism is still very much on the table.