This would be justified in a war context, but it’s being treated in a crime context.
We still trust the government with war, but also expect it to call war, war. Don’t lie to us that this is crime fighting.
Even assuming you're right that this would be justified in a war context, the problem is that, if this tool is created, the government would not hesitate to use it in other contexts.
Today, they want to unlock a dead terrorist's phone. Tomorrow, it might be a drug dealer. Or someone suspected of insider trading. Or Ammon Bundy. Or you, or me or anyone reading this.