Extortion is one of the better possible reasons for their DDoS. Eventually, when the money doesn't show, its in their best interest to stop attacking. While new extortionists will keep showing up, they won't likely come back.

Ideological or government attackers, on the other hand, can't be reasoned with and can only be expected to escalate in the future. If that's who Github is dealing with, then they'd better figure out how to actually deal with DDoSes sometime soon, because they will only increase in size and frequency.