You can make a case for classified/secret backdoors in times of war. Enigma was backdoored. The NSA used backdoors to crack does during WW2. The difference is these backdoors were top secret and were not justified to be used merely to do law enforcement but were to win a war. So from a practical perspective in a time of war it can be expected that all sides will attempt to create secret backdoors and crack codes.

But that has nothing to do with announcing the creation of backdoors and mandating backdoors in US products. All that will do is make it so the rest of the world cannot trust US companies or US products. Any terrorist would simply stop using US products knowing that our government mandates backdoors in them. So it would not do anything to stop terrorism or help in a war effort if it's known to the enemy.

On the other hand law enforcement would have it and whatever law enforcement can access it would also be possible for foreign intelligence to access. So what security do we gain by giving out backdoors for law enforcement? If it's about terrorism then statistically you are more likely to be struck by lightening, die in a car accident, etc. So the actual risk from terrorism doesn't justify the proposed reaction. It can only be a power grab.

Backdoors might or might not exist but if they do exist they should remain top secret. Additionally there should be no expectation for civilians to provide backdoors to help law enforcement. Private companies have their own missions and those missions aren't to be agents of the FBI. Finally, if Apple or any company were to be an obvious agent of the FBI then the targets wouldn't use their products which would reduce the influence of these companies and also hurt shareholders.

So whether you take a nationalist pro government or an anti government perspective the backdoor makes little sense. If China started putting backdoors in their products and announced it how would we react?