We're talking very long term here. It could be 50 years from now ! I expect to still be here (alive I mean). I agree that the maths look impossible today, but when deep diving into even just a regular CPU/GPU of today, not a quantum one, it seems impossible too, yet it works. Billions upon billions of transistors in a single chip !
If you're referring to the brute force cracking of 128 to 256 bit private keys, then it's more likely to take 100 years than 50. All the current math points to millions of years at the fastest possible computation speeds (and also impossible calculation speeds.)
What's more likely and probably is some crack at the algorithm that breaks it, so calculation is less than 128 bits.