I don't think it's true that 0 actions are being taken. When new vectors for amplification attacks are found, they get patched - you can't do NTP amplification attacks on modern NTP servers anymore, for example. But it takes a long time for the entire world to upgrade and just a handful of open vulnerable servers to launch attacks. And in the meantime people are always looking for new amplification vectors.

> The solution is to finally hold accountable attack origins (ISPs, mostly), so that monitoring their egress becomes something they have an incentive to do.

Be careful what you wish for. The sort of centralized C&C infrastructure and "list of bad actors everybody has to de-peer" that you would need to this effectively would we a wonderful juicy target for governments to go, "hey, add [this site we don't like] to the list, or go to prison".