The saying is a little outdated, or at least it needs more nuance rather than being a hard rule.
Nowadays lenses are improving in image quality significantly, particularly in the suppression of chromatic aberrations, and sharpness across the frame in zooms and wide angle lenses. Cheap mirrorless lenses put legendary L glass to shame nowadays. A bunch of Chinese manufacturers have autofocus lenses with amazing image quality. However, the bodies are sort of plateauing with 24 MP sensors still being popular for many years.
Nowadays lenses are improving in image quality significantly, particularly in the suppression of chromatic aberrations, and sharpness across the frame in zooms and wide angle lenses. Cheap mirrorless lenses put legendary L glass to shame nowadays. A bunch of Chinese manufacturers have autofocus lenses with amazing image quality. However, the bodies are sort of plateauing with 24 MP sensors still being popular for many years.