In the current Death Strike, Rune Strike incarnation you are 100% right. However I was theorizing about if DS is gonna use RP, and RS frost/unholy runes, then you can use the death runes from rites of blood on either 1) bloodboil/heartstrike/dnd for threat 2) death siphon for extra mitigation ON TOP of death striking from RP
I really think switching Death Strike and Rune Strike resources, and some small fixes to smooth things out is the way to disentangle the threat/mitigation problem.