After Archimedes, there was Apollonius (a mathematician in the same league as Euclid and Archimedes by consensus of historians of mathematics.)
Even later (no one knows exactly when) lived Diophantus. Apollonius and Diophantus created algebraic geometry in the same sense as Archimedes
invented integral calculus.
Whole science of trigonomerty was developed after Archimedes death.
These are mathematicians. But Greek astronomy even did not even start at the time of Archimedes: the first of the two greatest astronomers of antiquity, Hipparchus, was born after Archimedes death. The second and the most famous, Ptolemy, lived more than 300 years after Archimedes.
It is true that Greek science experienced the periods of decline.
But the final decline and the end of it
came only at the time of the late Roman empire. (Ptolemy or Diophantus would mark
the turning point).
You can certainly try to argue that Archimedes was the greatest of all of them. But continuing this line of thought you may conclude that the turning point of Western European science was Newton's death:-)