[eru]

I'm not sure what you mean by predicting prime numbers?

It's very, very easy to find big prime numbers: you generate a random number in the range that you are interested in, and then check whether it's prime. Repeat until you find a prime; they are fairly dense (a random number `n` has about a 1/log(n) chance of being prime) so you don't have to try too often.

In fact, that's how we find big primes for creating things like RSA key-pairs.

Testing a number for primality can also be done fairly quick. In general, much, much faster than finding the factors of a composite number. See https://en.wikipedia.org/wiki/Primality_test

> Has anyone ever considered a plan B for such a scenario?

Yes, quantum resistance cryptography is a thing. See the other comments.