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My understanding of SegWit is that any non-segwit (legacy) nodes will see a SegWit output as an output anyone can spend (i.e. no signature required). This is currently fine since if the miner mines a block with a transaction that tries (illegally) to spend from that output, it will be rejected by the majority of the nodes because most nodes support segwit and see that there is no witness proof as part of the transaction. But my question is how could a segwit output possibly be used in the beginning when segwit was just launched and the majority of the network where non-segwit? Then anyone could just spend the segwit outputs? Thanks!

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Segwit was activated using . The precondition for segwit's activation was that 95% of the hashrate was signaling readiness to enforce segwit's rules. Also, a majority of the other nodes had upgraded at that point. Quickly after activation, almost 100% of the hashrate was running a segwit-compatible version.

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  • Thank you so much! Are there any good reasons for why anyone would want to run a legacy miner these days? I assume they are open for attacks where someone can send them invalid transactions (with a high fee to make sure they are accepted to the block) and any block mined would immediately be rejected? Jul 31, 2022 at 14:48
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    I'm not aware of any reason to run a legacy miner except for ideological opposition to segwit. I do know multiple reasons against it: a legacy miner would not include any segwit transactions, since they look non-standard to a legacy miner. As today 5/6 of all transactions use segwit, they would earn less transaction fees (but fees are still only a tiny part of the block reward). Also the last non-segwit version of Bitcoin Core is severely outdated, it has been end-of-life (beyond maintenance patches) for more than four years at this point.
    – Murch
    Jul 31, 2022 at 23:50

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