The easy counter: "Except encryption works on maths; a mathematical "warrant" is more like a skeleton key that applies to every single letter ever sent and every safe ever made and every house ever built – and not one of those fancy keys that's hard to forge, because it's data; once you've seen one, you can just copy it and use it without anybody ever knowing. What happens if [terrorists] see one of these "warrants"?"
And to pre-empt: "And no, we haven't discovered the maths to make it not work like that. Nothing we know about comes even close, and such mathematics might not even exist."
A slightly more abstract counter:
> The government can get a warrant to read your letters. The law can regulate that, and there's no way they can get a warrant to read everybody's letters; only suspects will have their letters read. But these are computers. A computer could just go through, unlocking and reading everybody's letters, and looking for — look, imagine a future, slightly more evil government decides that puppies are illegal. A computer could look through everyone's letters for anybody who'd ever been pro-puppy, and then fine them for puppy-supporting, and then anybody who continues to support puppies would be put in prison. I know that puppies are a stupid example, but imagine that the government really wants to start a war, or something, and you disagree, and the election's coming up, and you can't talk about maybe not starting that war.
> Okay, so you trust this government, and all future governments, not to do that? Do you trust every single person who's ever seen a warrant? Say you want to get a job, but you can't, because you maybe don't want to save the pandas while children are starving in [impoverished nation] and you said so in a message to a partner three years back and your employer does a quick background check with some company somewhere and it comes back "[person's name] does not like pandas". It's your dream job, and now they don't want to hire you because the HR person is a really big fan of pandas. I'm picking innocuous examples, but come on, you can think of more significant ones. (And those background check companies already exist – they literally Twitter-stalk people; do you really think they'd ignore private conversations if they had access to them?)
> Do you trust anybody who's ever seen a warrant with your bank details? Sure, those skeleton keys will be kept fairly secure, but it only takes one person to make a mistake and then every single message that's ever touched a computer, past, present and future, is public.
> Or they could just look at the lock and figure out what shape of skeleton key would fit it. That's how all of the other backdoors the government put in computer systems like this were spotted. And once you've spotted it, you can just decrypt everything almost instantly, because you're just getting the computers to do maths and computers are fast at maths.