It's hard for the public to care when there is no concrete evidence of harm. The government knows this and is working hard to make sure that any harm done by mass surveillance is hidden using parallel construction.

What is an interesting and useful game is to imagine a scenario which would trigger massive public outrage over the use of mass surveillance. What would really drive the point home that this is a truly dangerous amount of power? To be honest, I don't think a single major mistake with a citizen would do it (e.g. killing someone based on mass surveillance). Too many apologists would make the case that this is cherry picking and that it's just an exceptional case. A pattern of mistakes would perhaps be better (putting thousands of people on the no fly list for no reason), but these kinds of stories tend to be too cerebral for the general public to focus on (although people like Colbert and John Oliver are really good at solving that problem!)

I think the only thing that might do it would be a Watergate style scandal where one group of powerful people use the machinery to mess with another group of powerful people. Consider how much more seriously Feinstein took the CIA when it became clear they were spying on her and her staff during the torture investigations. Or Angela Merkel's reaction to her cell phone being tapped. So, unfortunately have to wait for the machinery to be wielded against the powerful because it's (clearly) easy to get away with wielding it against the rest of us.