This patch makes the best case the same as the worst case: zero-confirmation transactions can be "overridden" if another transaction with a higher fee is broadcast before it makes it into a block.  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.e. users currently experience transactions not being replaced so readily, so they build around an assumption that zero-conf are okay for zero-trust situations.  They're not. 

I agree this is not inline with the original intention of the software, but I don't think there's a way around it.  Miners will do this eventually, even if most of them aren't, yet.  Let's not pretend they won't.

Plus, this patch comes with the nice benefit that "stuck" tx can be easily fixed.  Though, I do agree that a non-deterministic "undo" button is a bad idea.  It would be unreliable in many ways, and would also give a false sense of reality to users.  Instead, I'd like to see a button/dialog that says "This transaction has been waiting more than one hour without being accepted.  Would you like to increase the fee to try to speed up its acceptance?"