A backdoor into your computer today could be a backdoor into your brain tomorrow. Why create the dystopia of tomorrow with the unwise decisions made today? It's just not worth it.

Look into extended mind theory to understand why privacy should be protected. If the owner doesn't want to give their password or private key then it's just lost. In the case of the terrorists who killed themselves and we can't retrieve information from their phones? But if they are dead we can't retrieve it from their brains either so in a way it's an incentive not to kill terrorists and maybe it's best we encourage people to sell their data rather than use threats.

As far as backdoors go from a practical and less philosophical or ethical perspective, a backdoor is not effective at all once it's known. So a secret backdoor might be useful to stop terrorists but if it's known then it's just going to hurt everyone, including businesses, and not help terrorist investigations at all. So whether you take a philosophical ethical stance or a practical stance it is not wise to announce a backdoor.

In WW2 there was a backdoor in Enigma but it was not announced.