It's hard to evaluate the impact of a commencement speech.
George Marshall (1944) announced the Marshall Plan at Harvard University and changed the world in 20 minutes.
Tom Mboya, as a 29 year-old young African (Kenyan) leader in 1959, at Howard University helped inspire the American sit-ins by his presence alone. Walter Young was in that graduating class from Howard University's dental school.
Jimmy Carter gave the Morehouse commencement as governor of Georgia, and said as he received his honorary degree in 1975, that he expected he would be the first Morehouse man as president (of the United States), but "I promise you I will not be the last."
Obama by his own appearance, as President of the United States, in front of 10,000 graduates, family and alumni on a rainy day in Georgia fulfilled that promise, but may have also inspired a new generation of leaders to return to, and to rebuild their own communities; and their own families too. The president did a good job connecting to this new generation of young people.
In today's complicated political and economic environment, nothing truly significant can happen in just one generation. Obama lit a fire for generations to come.