I don't think there are multiple arguments here to beat around, despite people going on like it's a many-sided story. It all comes down to two conflicting principles:
- People's privacy is inviolable
- State's right to surveil people's actions must be unlimited
Before now, the balance was kept by surveillance being too expensive. But it was already pretty obvious in the 80s that we're quickly going full cyberpunk: communication and processing of info become dirt cheap, everyone is moving to digital comms for ease of use, and suddenly vastly expanded surveillance is easy, both on the net and in the physical world.
In ten years, net connection will be ubiquitous like electricity, all info about the world will be processed in real time, minds will directly control computers, and the agencies will ask why they should give up vacuuming it all if someone might plan a crime somewhere in there. Why draw the line at the datacenter instead of personal computers if the boundary is barely there? Why must there be a limit? The argument of “there might be something unlawful on there” doesn't have a limit.
If you think that a discussion between people, or their actions, should be private like they were before, you gotta ask where the firm line is. But I don't really see anyone doing a cost-benefit analysis on privacy vs surveillance, since conveniently for the agencies it's a ethics issue, and measuring ethics with numbers is frowned upon. So it's gonna be “X crimes prevented and Y solved” vs some indeterminate inconvenience caused by data leaks and corrupt officials.
As a bonus exercise, ask yourself: if to beat criminals the police has by principle to have criminals' tools―violence and disregard for privacy―then what stops police from turning into criminals on the side? These two markets are for the same skills. For some countries, it's not an idle question. And obviously, if a tool is available to police, it becomes available to criminals too.
But personally, I don't think privacy advocates will ultimately have much weight in the decision on this dilemma. People like to pretend that they highly value personal freedom, but the whole shtick of society is that it has a net benefit for a population by limiting individuals. Band together with other people, lose the freedom to be as gross as you want as loud as you want, have to do favors to keep connections. Pay some organized bullies to defend from other ones, concentrate on your own job instead. Move to the city, be highly visible to many people but have a variety of decent food, and sewers. We were giving up freedom for security and convenience for thousands of years, and I doubt we're going to stop now.
(BTW, afaik the cliché quote about giving up liberty for safety is used completely wrong and originally had exactly the opposite context.)