As if the slaves were going to survive that; in fact, they had more chances of dying than the celestials themselves.
Celestials were going to keep getting more and more slaves with the help of the likes of Red Dog Sakazuki as long as they were alive, who knows if the likes of Ginny would still be kidnapped or not later if Xebec was allow to kill many of them if not for Roger and Garp.
Roger only compensated that by creating the Pirate Age later, so he gets a pass. Creating the pirate age is a bigger blow to Government than killing few Celestials in God Valley. I am sure there were still more Celestials in Marijioa, and there was still Imu waiting there too during God valley.
But Garp is still shit, he didn't compensate anything.