An optimist would say you don't need to find all private keys, since many of them will lead to the same address.
No, but I want to demonstrate how many private keys there are.
The odds of finding a private key that corresponds to the same address, is 1 in the 2^256 / 2^96 = 2^160. Even if you used 1000 super fast computers that make a total of 1 quadrillion searches per second, you'd still need more than 8 nonillion (10^30)
days to cover half. If the age of the universe is 13.8 billion, then that'd be about 612,881,456,878,565,703 times the age of the universe.
So, yeah. We're pretty safe with that hypothesis.