The Satoshi v0.5.3 protocol views those P2SH addresses as scripts without any public key...

So the miners roll back their software to the version of bitcoin that can only recognize the P2PKH. The full non mining nodes who will still be running the later software then become part of the worthless Core fork unless they also revert. However, writing bitcoin code is more like designing hardware. Your bugs will be forever enthroned on the blockchain for the sake of immutability. One other potential thing to consider is the bugs that could result from such a rollback after hardware changes. I don't think there would be a problem with ASIC's yet since they have one thing to do which is find the nonce, but the longer segwit remains as a standard, the more likely other hardware changes will make it increasingly too difficult to roll back.

This is the only potential problem that I can see with your argument at this point. It's a timing issue. Imagine rolling back to Windows 95 on current processors. Most people wouldn't be able to comply. But it might be extremely profitable for the ones who do. What's the chance that a fork of Core from the bitcoin classic wouldn't be like Eth and Eth classic with Core being like Eth in marketcap? IOW, what other violations of economic principle have Core engaged in?