Privacy from the government isn't even the (main) issue. It's privacy from the people who will take advantage of the flaws in encryption created by enabling the government to have easy access to these devices.
Since we seem to like the door analogy, here is a decent one. You have a lock on your door, with a PIN Code to open it. You are the only person who knows it - you have it memorized in your head. Not even the company who made the lock knows your exact code. But then you write it down and give it to someone else who is ABSOLUTELY trust worthy. They would NEVER lose that slip of paper. But that point stands, now the code that used to be only in your head is written down on a piece of paper out in the world.
I just find myself having a hard time caring. Sure, search my phone just get a warrant first, like you would with anything else.
Let's say you sign up for a Thornton card. You save a few pennies every time you shop there. Meanwhile, they're storing a dossier on everything you've ever bought. "So what?" you say. Well, suppose you slip on a puddle of water outside Thornton's and decide to sue. Thornton's can respond by introducing into the record your habit of purchasing alcohol and paint you as an alcoholic. It's happened in an arbitration case in the US.
Maybe in college you signed up with Amnesty International because the girl at the table was cute. You never went to a meeting and you forgot about it. Let's say ten years from now AI is somehow linked to the humanitarian work done by Hamas and the Conservative government in power labels Amnesty International a terrorist organization. Then, in a pageant of nationalism, some government official decides that everyone who has any ties to terrorists should be compelled to explain themselves to the CIA. In private.
Don't know if that's happened in the UK, but it happened in the US twice.
I could go on. The basic issue is that your behavior at the moment may be completely harmless, but at any point in the future, anyone with an axe to grind against you or anything you've touched can use your behavior against you, out of context, on the offensive, simply because the information is available. Say, for example, you meet a nice girl. You fall in love. You want to marry her. She's got an old flame who she dumped because he was a nasty fuck, but he still holds a candle. Suppose he finds your Reddit username. Downloads a compilation of your GW submissions, prints out a stack of color copies and staples them up around your neighborhood, mails them to your grandma, etc. Have you done anything wrong? No. Has he? Yes. Is he going to be punished? No. Is your relationship with your true love in jeopardy?