Why does the spiral pattern of the Galaxy rotate as a whole?

According to Kepler’s third law, stars farther from the center of the Galaxy also have a longer orbital period. Therefore, the spiral pattern of the Galaxy should be destroyed, according to estimates, during about one revolution.
There is currently a theory that explains the preservation of the spiral pattern during the rotation of the Galaxy. Spiral branches are places of formation of young hot stars in areas of compaction of matter, which correspond to the approaching wave front. Thus, the wave front moves as a whole, and the stars, leaving the front, reduce the radiation intensity.