I mean to be fair, even without the instant kill a Kuzushi-Zantetsuken would still be 30,000 damage in an AOE with a dash - which I think would be fine in a world where Skyshatter wouldnt crank the damage to 40,000 from the inner ring and with a 20,000 outer ring hit (note: all under effect of Geirskogul which is standard practice).

I personally would not be opposed to removing the instant kill component as long as the Guard pierce on Kuzushi targets retains and the damage being amped in general.