Individual DHT nodes should only see a trickle of packets. A few kilobytes per second. Even less per remote IP. So they can set fairly strict rate limits.
And ISPs are in a much better position to solve this problem anyway. They should ask for higher peering fees from peers that don't do source-filtering.
True. Although an ISP can say they do source filtering and then fail to implement it properly. For example my ISP at home implements source filtering on IPv4, but not on IPv6.
And ISPs are in a much better position to solve this problem anyway. They should ask for higher peering fees from peers that don't do source-filtering.