In the past, a human was required for most policing operations. Economically, that meant you'd need to be a person of interest or at least incidentally connected to one to be subject to an investigation. It meant that judges and legislators could say stuff like "it's fine for the police to run arbitrarily license plates, that's public information" because running license plates meant there was a cop with a pad of paper walking through a parking lot, bringing the notes back to a clerk to manually tab through the registration files. Storing the data and processing it used to happen at human speed and cost on the order of $20/hr, assuming they weren't working overtime rates on nights, weekends, and holidays, now it runs at gigabits per second and costs $0.05/hr to run 24/7.

I'm not in the know, but I know that all the email content I've ever written is trivial to fit on a $50 hard drive, and with that I can search through 20 years in under 20 seconds.

It's not a question of "could they do it" but "would they do it", your answer to that question depends on social and political issues, not technology. It's why I prefer technical solutions like encryption to political problems like privacy.