It destroys the chain of evidence. The government can not document what is being done to the phone to get the data, therefore no court on the planet will allow that data to be used. Only if the government does the hacking and retrieves the data itself can they claim to have a clear chain of evidence. There is no “compromise” that involves the government not having possession of the phone.
Maybe if the government actually kept the damned terrorists out of the country, they wouldn’t have to go begging for cellphones.
OK.
But let’s say Apple gives them a ‘key’, and they get data.
Then someone challenges the key, saying, “That key distorts the real data, making our client look guilty when he is not.”, or some such variant.
Just Slouching Towards Nauseam.