Dead serious. The mentality that everything you want to have needs to be explicitly justified to society before you are permitted to have it is a sick twisted authoritarian mindset. I thought our society was better than this
2) The same reason I support the second amendment. The government is gigantic and powerful and scary. Even if it acts in the most benevolent way possible, it is gigantic and powerful and that is _intrinsically_ scary. The government can _fuck up_ and destroy ten thousand lives before anyone even notices. Consequently, people need ways to defend themselves from the government proactively. Encryption is one such way.
2b) If someone wants to argue that "what if criminals use it to do crime", remember that marijuana is still a federal crime, and some absurd percentage like 30% of all Americans have smoked it at least once. It is well within the government's power to just spider through all social media to see all references to marijuana, use that as probable cause, and do raids on _MILLIONS_ of people. Will this happen? Almost certainly not. COULD this happen? Absolutely. Unless, of course, all those communications were encrypted such that nobody could access them. I don't think "I pinkie swear I won't do it" is a good enough protection for me against that possibility
3) the cynical answer: we already have ample evidence of actual child sexual abuse rings, but for some bizarre reason the authorities lost interest in following up on that once the ONE guy they got hung himself. If they aren't willing to do the police work on this issue that they already can, I don't see what the argument is to give them full access to all crypto systems.
4) Technical answer: Just because you make a backdoor and give the government the only key, doesn't mean the government is the only people who are going to use that door. Maybe they lose the key. Maybe they give the key to someone who turns out not to be trustworthy. Maybe someone makes a secret copy of the key. Maybe a burglar doesn't actually get the key, but he's really really good at picking locks and so the backdoor makes it that much easier for him to get in. Security is a hard problem and every single compromise increases your risk surface area. The first lesson of security is "assume the worst possible thing happens, and then prepare for something worse than that". Such a back door (or, alternatively, legal prohibition of e2e encryption), dramatically compromises security simply by existing.
5) The tinfoil hat answer: The fact that they want it so badly tells me that they shouldn't have it
6) The current year answer: Do you want Donald Trump to personally have the ability to spy on anything that you, specifically, do? Y'know, if he's bored one day and wants to find something stupid to tweet? Do you want him to have that power? I don't