Unbreakable cryptography isn't real either. It's not impossible to reverse engineer a private key it's just impractical. It would require immense computing power and only yield that individual private key broken.
What the president is proposing would allow a key for law enforcement that would, presumably, work on many or all devices. Once cracked, it would give hackers access to all of those devices. This elevates risk considerably because it's no longer impractical to reverse engineer the encryption.
"If the key is truly random, is at least as long as the plaintext, is never reused in whole or in part, and is kept completely secret, then the resulting ciphertext will be impossible to decrypt or break"
That's not a great example in this case, because any useful system that incorporated OTP would have to store the really long secret keystream somewhere. That somewhere would be vulnerable to subpoena.
When that immense computing power is such that it would require turning the entire observable universe into computronium dedicated to the task, and running it for longer than the heat death of the universe, I feel comfortable calling it "unbreakable."
This debate is not about the direct ask imo, but more about the precedent that it would set and what the fbi could compel a tech company to do under the AWA. While in tech we tend to turn up our noses to the 'its always been done that way' mentality, the law takes a different approach, once you comply with the first request, it makes it harder, if not impossible to stop the subsequent requests. Which is why the fbi waited until something related to 'terrorism' to make this ask. I would be willing to be every last dollar I have in the bank that there is no useful info on that phone, and that if there were useful info, the nsa would have already had access to it.
To me, the really fun mental exercise is to identify when this would be acceptable. If there was a bomb about to go off and hypothetically the location was written on the phone, and only on the phone, would we agree that apple should break in? What if the bomb was a dirty bomb, what about a nuke? Now the slippery slope is where does that stop, a backpack bomb? Bomb making materials, texts to people about a bomb, what about a gun? What about a knife....
What the president is proposing would allow a key for law enforcement that would, presumably, work on many or all devices. Once cracked, it would give hackers access to all of those devices. This elevates risk considerably because it's no longer impractical to reverse engineer the encryption.