Zero-conf replacement requires only a few miners to participate for it to make zero-conf transactions pretty much useless in zero-trust transactions.
I don't think that's right - people accepting zero-confirmation transactions are already playing the odds. 1% of mining power taking the later arrival with the higher transaction fee still leaves you getting the payment 99% of the time, so in a lot of cases it would still be worth it for the extra sales.
Double-spend attackers welcome, buy 99 pizzas and get one free...