Correct me if I'm wrong, but if P is not equal to NP, then we ought to be able to derive as much entropy as we need from a key which is non-brute forceable (at least for symmetric encryption). Eg, if we believe 1024 bits of entropy cannot be brute forced, but our algorithm requires a 4096 bit key to provide at least 512 bits of security against cryptanalysis (plus a margin of safety), then we can derive our larger key from our smaller key without sacrificing any security.

But there's an implicit assumption here that all keys are equally strong, so this doesn't apply to asymmetric encryption. At least not as straightforwardly. And it's possible that P is in fact equal to NP. And there's a bunch of other assumptions here too, like that we really do have a secure source of entropy and really can share keys securely.

Anyway, if we take all these assumptions as read, this suggests that symmetric key lengths will saturate at a certain point (and not much wider than they are today). Big if true.