The real reason why UAHF spec won’t require replay prevention.

Instead of making their new sighash optional, they could easily make it mandatory and prevent unintentional replays…

I think I found out why.

1. Alice is a long time hodler of bitcoin, she sees that segwit has done great things for bitcoin and decides to move her coins from her old paper wallet to a Bech32 scanned P2WPKH address that the new Trezor interface (in the future) let's her send to. The paper wallet sweeping app can send to P2WPKH properly. So she is able to simply send her 100000 BTC to her shiny new Segwit Trezor… yaaaaay!
2. Bob sees the transaction being broadcasted without any OP_RETURN nonsense, and smirks to himself.
3. Bob takes the txid of the transaction Alice sent and the index of the segwit output. Slaps on a null scriptSig, and adds an output spending the whole entirety of the output to his P2PKH utxo, and broadcasts Alice's transaction AND Bob's new transaction on the BCC blockchain where Segwit will not be activated, so Alice's P2WPKH is merely an ANYONE_CAN_PAY utxo.

So basically, any users that wish to use Segwit after the lockin period in late August will need to be certain to include the stupid childish OP_RETURN replay protection OR face having your segwit funds be stolen on the BCC side.

Also, to take it one step further, a miner could easily just steal the output and put its entirety into fees which they would use to line their own pockets and mix the coins in to make it look like the miners themselves didn't steal it.

You know, I was starting to support UAHF's actions because they seemed to be amicably splitting off and saying their goodbyes, though I thought it very strange they were making replay prevention optional……. then it hit me. This was their intention all along. A final middle finger to Segwit users.

UAHF needs to remedy this. Or else they lose all respect.