What does the broken statue of Ozymandias symbolize?

The broken statue of Ozymandias symbolizes the false power a tyrant holds.
Ozymandias had a huge ego and thought he was magnificent and great. He also thought that the empire he built would last forever. He had a huge statue of himself carved, showing a sneer on his face and bragging to those who might dare to challenge him to

Look on my works, ye Mighty, and despair!

However, nothing is left of Ozymandias's great works and kingdom but a broken statue and endless desert sands.
Shelley was a radical and a republican, meaning he supported elected governments rather than hereditary monarchies. For him, Ozymandius symbolizes how tyrants fail. What other tyrants should be frightened of is not the great Ozymandius, who is, after all, nothing anymore, but that they will suffer his same fate. They too will be obliterated by the sands of time.