I don't understand how you could ever rely on a newer transaction overriding an older one unless unlocked transactions make it into the blockchain in a "passive mode".
You can't. If replacement was functional in the network you could pay a higher fee and hope that this incentivizes the next miner to do the right thing, but it's not a guarantee. (And I can't see how any amount of 'passive mode' could really help there, except perhaps by making sure the sequence number cranks only in one direction, but someone could still fail to include a newer one).
