We are arguing that this unquestionably how miners will operate in the future, anyway, so we change the default now to prevent any further adaptation to a false sense of security/reality.
I doubt it. If there is ever a significant zero-conf market (especially a brick & mortar), they can pay mining pools to NOT allow replacement for TXNs originating from them.
This would reduce the likelihood of a double-spend dramatically.
As the credit-card business aptly shows, problems not theoretically solvable are often 99.93% (7 cents out of 100 bucks) solvable in practice
http://en.wikipedia.org/wiki/Credit_card_fraud