The end of the novel shows both the despair at the war that seemingly could not have been averted, and the hope that Granger, Montag, and the rest of the Book People can bring to civilization. The banning of books was done to prevent individuals from developing opposing opinions and fighting; clearly, this approach failed. Now that the city has been destroyed -- along with countless others across the country -- it is possible that the next civilization will be more enlightened.
"And some day we'll... dig the biggest grave of all time and shove war in and cover it up. Come on now, we're going to go build a mirror-factory first and put out nothing but mirrors for the next year and take a long look in them."
(Bradbury, Fahrenheit 451, Google Books)
Granger's point is that humanity cannot continue to evolve and grow if they try to cover up and hide the past. Banning books allowed the government to hide the truth about history from its citizens. Now that the government has been destroyed or crippled, the future is brighter for individuals, who can make their own decisions and who are not required to walk in lockstep with government thinking.