> it might be nice to get a warrant for his phone and see who else he was talking to or conspiring with.

The bar for developing law and schemes for eradicating privacy rights should be higher than "it might be nice".

The bar is: "this man is a known terrorist and has committed a domestic terrorist attack; therefore we have grounds to search his private records to search for co-conspirators, accomplices, etc."

That's a pretty high bar if you ask me.

I'm sure they'll get a warrant for his phone and have a look.

The problem is that we have already extant and widely circulated encryption algorithms that (we believe) are secure against decryption. It's math. You can't "uninvent and destroy" all the math and preclude its use by adversaries determined to use it.

True, but you can make it more difficult for normal people to use. You can't uninvent fully automatic firearms, but you can make them difficult for normal people to obtain such that they are effectively never used (even by determined terrorists plotting mass shootings)

You are comparing harmless messages to dangerous weapon. It would be ridiculous if one could buy a rifle with a range of hundreds of meters but couldn't use an algorithm to encrypt a message.

Who says messages are harmless? I disagree vehemently and contend that information can be indeed more harmful than firearms in the hands of the right people.

A single message can mobilize millions of people to take to the streets and riot. A single firearm can't do that.

A message sent to millions of people will be either public or will be leaked. So it doesn't make sense to ban encryption for such messages. Better ban riots. Also, there is something wrong in the country if millions of people are ready to riot.

Well, there was this: https://simple.wikipedia.org/wiki/Assassination_of_Archduke_...

Was it the firearm that did that, or the propagating information regarding whom was killed that did that?