These companies have made themselves extra vulnerable by the fact they all have very low TTL A records, so dns lookups wont be cached for long.
While this could be seen as a vulnerability, it is also an advantage, because as GitHub have done, it aids in the transition to a new provider in lieu of their current provider being attacked.
In terms of this attack, the fallout initially will last a few days I feel, but we will probably see in the long term a lot of companies moving to in house DNS, or moving to providers that actually use DDoS protection in front of their servers that are capable of defeating such large attacks.