Barr's point is that it's better to have that argument now, in a level-headed moment and with opportunities for all the relevant stakeholders to provide input, than it would be to have it in the middle of some dire emergency.
If you oppose back doors, I would think you would agree with him on this -- government tends to be delegated sweeping powers in emergencies, so it would be much harder to stop gov-friendly proposals like "back door all the things" in that kind of moment than it would be to stop them now.
"We must do something now, it's clear that we need backdoors because of X!" <- That's a much worse position to be in during a debate.