For one, Current Kaido is just as durable as he was back then and he has the same durability both in Dragon form and hybrid unless specified otherwise. I won't deny he might have a better endurance now and I also won't deny Kaido is more powerful now than he was back then. But Oden had Kaido at his mercy with Togen Totsuka.
Two, Zoro doesn't need to be as strong as Oden to have a Togen Totsuka level attack just like how Law and Kid need not be individually as strong as BM to have attacks on par with her. Zoro does hit Oden level in King of Hell, which is temporary. There is no reason to draw a parallel b/w the two attacks if it's not meant to convey the implication that the two attacks are similar.
Law's attacks are deadly because they ignore durability which is what makes him hax. You're just describing them differently to make it sound scarier but it isn't as scary as you think it is. Sure Law's attack may be deadlier but it's nowhere near as strong.