If there was an attack to reverse Segwit, the network will hard fork and the chain will split into two. The mainCore chain will behave like nothing has happened while the forked real Bitcoin chain will reverse Segwit.
I made a slight correction to your summary. Core will actually fork off from the original Satoshi version 0.5.4 protocol at that juncture. As a matter of correct definitions and semantics
It's not a question of semantics. The chain with the most people behind it is Bitcoin. Anything else is an altcoin. You don't get to pick an arbitrary point in history and declare that "
nothing beyond this point is the real Bitcoin", because that's not your sole decision to make. Again, 0.5.4 is an outdated client run by a small but vocal number of hardline fanatics. That's a speck in the wind compared to the combined will of an entire network of users all over the world who couldn't give two shits what you or MP thinks. The moment a fork occurs and 0.5.4 isn't compatible with what everyone else is running, 0.5.4 becomes the altcoin. They can have their Thiefcoin, the rest of us will ignore them and have Bitcoin.
You don't decide what the "real Bitcoin" looks like. That's not how consensus works. You've been duped if you think otherwise. But then that's only natural, because that's what drinking the trilemma kool-aid does to you.
But you forgot a very important point I made several times already and you continue to forget. That is those who hodl legacy addresses that begin with 1 will receive both real Bitcoin tokens and Core tokens. Whereas those who hodl in addresses that begin with 3 will receive only Core tokens. Even if you support Core, you would want the real Bitcoin tokens so you could sell them and contribute to fighting against it and use the proceeds to buy Core tokens. But to do that, you must forsake SegWit in the interim time.
You may have noticed that any fork which proposes a fundamental change to the founding principles doesn't even get off the ground. Your fork would be held in the same esteem as forks that propose raising the 21 million supply, bringing "lost" coins back into circulation, anything to do with blacklisting certain addresses. People who appreciate Bitcoin for what it is don't give those projects the slightest bit of notice. They're irrelevant. As is Thiefcoin.
If you have coins held in SegWit addresses, you would indeed fight against the 0.5.4 chain, but you wouldn't do it by buying 0.5.4 coins to dump them. It would be contrary to your own financial well being to give even the slightest hint of recognition to the chain trying to steal from you. The best way to combat a chain like that (which is clearly an affront to the underlying principles of property ownership and crypto in general), would naturally be to tell everyone to
ignore the 0.5.4 chain completely and leave it to rot in the gutter. There wouldn't be an actual economy on that chain and it wouldn't be sustainable. So all that would remain is a niche blockchain where the sole purpose is to effectively quarantine the insane from the rest of society. As such, I eagerly await your fork and you disappearing off to the land of make-believe to play with the other inmates in the asylum.