Bridges deteriorate by default. That is a well understood process with well understood reasons, which we monitor and work against. Salt, thermal cycles, load, rust.

Humanity does not deteriorate by default. Claiming it does so through some hand-wavy pseudo-evolutionary arguments is not a strong case, and requires at least some evidence to be taken serious. How about a (equally unfounded and just for the sake of argument) reverse claim: Humanity got more intelligent, because high child mortality favored physically strong children instead of mentally strong children.