It's actually sad because that was the most interesting part of his character.
Kaido being invulnerable worked way better for him than the whole "strongest creature" thing because it made him stand out.
It would have made him look scary in an equal but different way as opposed to standard "World's Strongest" types, giving him his own unique niche as a top tier.
(As it stands the WSC title feels less legit as WB/Roger/Garp were all established "world's strongest" types long before Kaido appeared on screen)
Paired with a good backstory, he could have been a character you could have loathed and sympathized with at the same time - a Shin Godzilla type tragic antagonist who destroys the world around him out of rage at his own inability to die.
It would even have fit Luffy's liberator schtick by letting him kill Kaido and free him from his curse of life.
The problem I think is that Oda wanted G5 to be his Super Saiyan and decided to make Kaido his Frieza, and that complicated things. So now you've got Kaido the Conqueror who is also Kaido the man who wants to die, and that just makes him inconsistent.