Just one breach is all they need in order to open the floodgates. We have seen not only a loss of data, but also a corruption of that data.
It's not clear what Mueller means by a corruption of data, but security experts warn that if attackers are able to change source code, they might add back doors or logic bombs in the software they access.
Certainly, Mueller is concerned:
If hackers made subtle, undetected changes to your code, they could have a permanent window into everything you do. Some in industry have likened this to death by 1,000 cuts. We are bleeding data, intellectual property, information, source code, bit by bit, and in some cases terabyte by terabyte.