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You don't need the whole network to support transaction replacement to do that; just write some code that holds a time-locked transaction, has a way of replacing it with another time-locked transaction, and have it automatically broadcast the transaction onto the network if not replaced before the time-lock expires.
It would be more convenient to have the whole network support transaction replacement, but I don't think it is reasonable for the whole network to remember everybody's dead-man-switch transactions. And even if they did, you'd run the risk that a miner eager to get an extra transaction fee would ignore a newer version of the transaction and mine the old transaction.