Do you mean there cannot be a CSPRNG? Most of cryptography is based on security in practice, not in absolute theory (i.e. everything except for one time pads and maybe some quantum crypto). That's like saying there are no cryptographically secure encryption algorithms other than one time pads, since you can break all of them in theory. That's a pretty useless definition of cryptographically secure.
edit: By in theory I mean that with enough computation resources you could break them even if you didn't find some new, clever weakness. Not that you could break them in theory because a weakness could always be found.
No, I mean that the fact that /dev/urandom "reseeds" in mid stream means it is not strictly speaking pseudo-random, since it is not completely deterministic. Maybe I'm wrong, but the comment I was replying to was arguing that /dev/urandom was the only CSPRNG, and things like stream cipher algorithms are not.
edit: By in theory I mean that with enough computation resources you could break them even if you didn't find some new, clever weakness. Not that you could break them in theory because a weakness could always be found.