I don't think he means existing designs shouldn't be reused. I think he means that you shouldn't lessen testing because you are using an existing designs. Boei

I don't think he means existing designs shouldn't be reused. I think he means that you shouldn't lessen testing because you are using an existing designs. Boeing reused the 737 design to produce the 737 Max; however, because it was a reused design, Boeing did not go through the same certification process or testing that would have been done for a new airplane. Skipping testing because you have a reused design is the bad habit he is referring to.