But isn't that precisely what this proposal is intended to deal with?
so he would have to mutate every possible transaction in order to get a mutant mined instead. This would be much harder/more costly (at least if P2SH were widely used).
This proposal assume that Bob would not take a spray-and-pray strategy. As someone is now randomly mutating transactions on the network, the proposal won't work
My question a year ago has not been answered. As someone may attack the network by randomly mutating txs, just like what we had last year, no txid should be considered as final before it is written in the blockchain.