"But my job is to try to keep people safe. In universal strong encryption, I see something that is with us already and growing every day that will inexorably affect my ability to do that job."
In a attempt at charity, I shall assume that Director Comey is an honest, trustworthy, diligent, well-meaning, moral soul with a genuine desire to protect people.
Many times, the argument runs that the people pressing for these measures are inherently untrustworthy and do not have the best interests of the people at heart. I think that approach is, though sometimes accurate, unhelpful, because such a claim will never sway those making the decisions.
The real problem is that these people do not understand that, not only is "keep[ing] people safe" not the most important consideration in a free and democratic society, even if it was, it is far from obvious that all-pervasive monitoring by the government and law enforcement agencies is the best way to do this - or even helpful, in the long term.
The problem is that, even viewing these people in the very best light, they believe that preventing a terrorist attack or arresting a drug dealer, or collaring a pedophile (our trio of witches du jour) is so important a goal and the benefits to society so great that they outweigh whatever ills are committed in pursuit of that end and justify whatever means are employed - no matter how infrequent and isolated the positives and how constant and pervasive the 'compromises' are.
They also seem to believe that stopping a crime or saving a life now is more relevant than any unintended consequences or long-term effects that such compromises may bring about.
One could also argue that the technology simply doesn't work that way but to do so is to, again, somewhat argue the wrong point because, just as it is possible (if difficult!) to imagine a situation where the people administering these schemes were beyond reproach, so too is it possible to at least hypothesise about the technology being available to make this work 'well'.
The issue is that, even if the government and agencies were everything they claim they are and the technology was everything they assert that it should be, it still wouldn't be a good idea and the downsides would still outweigh the potential positives.