Big instances would instantly lose every instance other than Meta's. Why would they ever do that? Especially when the biggest instances are mostly approaching Meta with caution if not already defederating?

The fediverse doesn't handle rogue actors all that well if they change the literal language used to communicate. It breaks it. Even Microsoft, the one who basically invented the strategy decided against it in the end. They broke the internet for a long while, but only because they were in a much stronger position than, say, Netscape, which was originally designed to fail anyway.

Meta would need to change the actual protocol if they wanted to get anywhere. Even then, instances can simply refuse to upgrade.

It is much more difficult than you think. And id much rather users see they can live without Threads than to force them into Meta's corner. I'd rather help people get away rather than ignore their existence. If Threads is too annoying overall, I'll vote defederate. But I want to see what happens first.