When an argument keeps changing shape, it usually stopped being about the argument a while ago. Someone who treats a position as part of who they are will experience a lost point as a lost self, and the mind responds the way it responds to any threat. The example changes, then the standard of proof changes, then the person across the table becomes the topic, and finally the tone of the conversation gets graded instead of its content. Every one of those moves protects standing, and none of them tests the idea.

I've come to think this explains most of why online debate feels so slippery. Both people believe they're arguing about the subject, but one of them is defending a position that has been quietly fused with their identity, and you can't dislodge it with evidence because the evidence is aimed at the wrong target.

The practical answer is to find the one principle underneath everything you've said, state it plainly, and keep returning to it without heat. Examples can come and go, but a principle either gets answered or it doesn't. If it doesn't, you've learned something, and so has everyone reading along