You had no right to privacy on a telephone line, unlil eventually Congress passed a law against wiretapping and set controls on when the government could wiretap, controls which were later strengthened by the Supreme Court.

Cell phones: Started off with no privacy. If you had a radio scanner you could legally intercept conversations, even if you were the government (after all, they were being broadcast right in the clear in plain view of anyone who had an antenna). Again, Congress passed a law to ban deliberate interception of these communications and set similar control s on the government.

Even beepers had a specific law passed for them.

On the other hand, your slippery slope argument has itself been used to support horrific policies. Does anyone remember the reasoning for Vietnam? It was based on a slippery-slope argument itself.

The flipside of having a democracy is that people might actually choose to entrust the government with a given power. For example, the American people have seen practically no qualms whatsoever about the American government having things which are much more potentially dangerous: nuclear weapons.