There won’t be any Segwit spend to any attacks…

There are barely any nodes running satoshi’s old codebase, that code was buggy and vulnerable and couldn’t handle higher traffic, so they’re all gone.

It’s the same as breaking any other consensus rule — like allowing joe-blow to spend block 1’s coins with a key that isn’t the right one, or mining 1000 BTC in a block or whatever: all nodes enforcing the rule will ignore the invalid blocks just like they never happened. Any nodes not enforcing it, if there are any, will play along so long as there is more hashpower mining the invalid blocks. Of course, the miners making invalid blocks are burying millions of dollars a minute in power to make “blocks” that pretty much only their own nodes will accept, so they’ll rapidly go bankrupt.

This is the same deal for any nearly-universially deployed consensus rule. Segwit would only be different if it weren’t deployed everywhere but it (like almost all other rule additions made going all the way back to the rules satoshi added in 2010) didn’t get activated until an overwhelming majority of users, all major businesses, etc. were enforcing it.

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