Right, but isn't "impossible" too high a bar? Or is the ability to comply with a warrant worth nothing?

It's not like iOS is impossible to hack now and it would be terrible that it becomes possible. There are other aspects of the system that allow malicious exploits more easily than this theoretical threat. So it doesn't make sense to preserve at all costs (e.g. making warrants unenforceable) an "impossibilty" that never was.