First you state that PRISM isn't used domestically. Then you admit that it is used domestically. I have no idea why you're mentioning warrants from the 1800's. "The feds were getting warrants to open mail in the 1800's." Are you trying to suggest it is used only infrequently?

While the number of times PRISM is used domestically is unknown, the attempts to keep the number unknown is eyebrow raising.

"Wyden repeatedly asked the NSA to estimate the number of Americans whose communications had been incidentally collected, and the agency’s director, Lt. Gen. Keith B. Alexander, insisted there was no way to find out. Eventually Inspector General I. Charles McCullough III wrote Wyden a letter stating that it would violate the privacy of Americans in NSA data banks to try to estimate their number." https://www.washingtonpost.com/investigations/us-intelligenc...

You do not need a warrant for international communications performed by non-citizens (whether inside or outside the country)

You do need a warrant for communications done by citizens domestically. PRISM did not change that, it has been the same for centuries. So even if PRISM captures domestic communication, a warrant is still needed to view it. There has never been a time in US history when then feds didn't have access to warrants to tap communications.

PRISM is precisely a submission & storage system for data requested via warrants. People have the impression that it's a backdoor, but that's not what the Snowden docs actually described it as.

The "backdoor", if any, would only be the rubber stamp courts that allow pretty broad warrants for plenty of dumb things, but that could absolutely be fixed if half the country didn't totally buy into "I have nothing to hide" bullshit, and were as militant about defending the 4th amendment as they were the 2nd, but I guess the inalienable right to privacy just isn't as cool as putting a hole in a steel target 100 yards away with a machine.