Open theism means God changes His mind and redirects future events based on the actions of man. Since you believe, apparently, that man controls the origin of the Antichrist, I’d say you’re an open theist. Most of us understand that Antichrist will rise regardless of what man does. So secession has no effect either way. It’s coming.
While there are examples of God changing his mind because of the actions of man, they are the rare exception, not the rule. And that's not at all what I'm talking about here.
What I'm talking about is the toppling of dominoes. The antichrist isn't going to just wake up one morning, decide he wants to take power, and have the world's leaders give it to him. For him to take power, there has to be a power vacuum, which is brought about by the collapse of the worlds governments, which is initiated by the collapse of the US government. None of this happens all on it's own, it's one thing leading to another leading to another leading to another.
You need to see this not as independent events, but as a chain of events. The antichrist doesn't just appear out of thin air. There is a long chain of events that lead up to him, a big link being the fall of the usa.
It will happen. The question is, do you really want to be a part of making it happen?