Apple has an enormous investment in their design team in Cupertino. It would be an enormous impact to their product development capability to start over somewhere else.
It's not enough to say "HQ is over here bro," court orders still work in California. Then again this whole All Writs effort to "build me a tool to help my investigation" seems to break new ground. Maybe it wouldn't even be enough to pack up and move all R&D out of the US (e.g. injunction on US sales until the foreign Apple company complies).