The very foundation of the Constitution is based on respecting the individual. Whether it be through speech, religion, the right to bear arms, the right to be secure in your home and your papers, or any other of the inherent rights to which we all are entitled. Nobody gives up those rights until after they are convicted of crimes in a court of law. As a citizen I am under no obligation to make it easy for government to track my conversations or beliefs.
Requiring a backdoor into the methods of engaging in conversation is a direct assault on free speech. Once it is known the government can access all of your transmissions it will have a chilling effect on people with viewpoints that differ from acceptable norms. Once the backdoor is in place do you really think it will be limited to only one alphabet agency? Of course not, it is inevitable that other agencies will provide their own justification for accessing that information.
For the same reason I would never allow law enforcement to install microphones in my house to capture all my conversations I won't allow them to install a backdoor into my phone to do essentially the same thing with my electronic transmissions.
The argument that some bad things may be prevented is more than offset by the protection of liberty. A bad person with the keys to everyone's private conversations and data is much more of a threat than a terrorist. All you have to do is look at Hitler, Stalin, the Kims in Korea, Hussein, Ghadaffi, Pol Pot, Amein, Pinochet, Papa Doc, and any other myriad of ruthless dictators throughout history to see where excessive power in government is the biggest threat to the lives of multiple millions of people. Just imagine what any of those individuals could have done if they had unfettered access to every dissenting conversation and the technological ability to sort through them quickly.
