Also, the people pushing for these measure (e.g., the U.K's equivalent of the NSA, GCHQ and most national-level police departments) understand these issues perfectly well.
Surely some of them understand the technical details. That doesn't necessarily mean they understand or respect the wider implications of a policy. This is why it's important to have a government that sets policy - taking into account all of the competing influences and potential consequences - and politically neutral technicians who then implement government policy.
No-one would dispute that if the government could examine every communication everyone ever sends then it could catch more very bad people and prevent more harm to innocent people. The problem is all the other stuff that also happens if you give a government that kind of power over its own people.
Also, the people pushing for these measure (e.g., the U.K's equivalent of the NSA, GCHQ and most national-level police departments) understand these issues perfectly well.