Because the problem P = NP is not solved, being able to reduce a known NP-complete problem, Π2, to another problem, Π1, would indicate that there is no known polynomial-time solution for Π1.
If P = NP is unsolved, and reduction is applied to a known NP-complete problem vis a vis Π2 to  Π1, what conclusion can be drawn for Π1?
there is no known polynomial-time solution