[Hendrikto]

> A CSPRNG is just an algorithm that uses some entropy as a key and feeds back on itself to generate a stream of random bytes.

You say yourself that there is a distinction. CSPRNGs can generate unpredictable ("random") bytes, given entropy. They cannot, however, produce entropy. They are entirely deterministic.

Entropy gas to be gathered. It cannot be generated. Therefore, there is a meaningful distinction between the input entropy and output random bits.

I do agree that once the PRNG is sufficiently initialized, it does not make sense to say that “entropy ran out”, however. Given a 256 bit key, modern PRNGs can generate unpredictable bit streams of sizes that will practically never be exhausted.