What is the correct mindset for Requirements Engineering regarding specification completeness?

Don't ask "is the spec complete?" — ask "how much must we invest to shrink the risk to an acceptable level?" Completeness is never the goal; managed risk is.

The naive framing treats a specification as something you either finish or don't, which leads to the excuse "we don't have time for a complete specification." The mature framing treats specification effort as a risk-reduction investment you dial up or down.

Rule of thumb: the effort spent on Requirements Engineering should scale with the risk at stake. A throwaway prototype needs little; a system whose failure costs lives or millions needs a lot. You stop investing when the residual risk is acceptable — not when the document feels "done".