Oden told Kaido to "build his strength". This Kaido is meant to be stronger than the Kaido Oden fought.
- Kaido already had his invincible hype 20 years ago
- He appears to have been a Yonkou for several decades
- Kaido placed Oden on par with the Greats
Oden is a top tier — that Zoro and co are fighting a stronger Kaido doesn't change that — it just lowers the bar on how far Zoro needs to push Kaido by himself for Kaido to acknowledge him.
All of these are tied into Kaido's desire to die at the hands of the Samurai bearing Oden's mantle. These come with an expiration date on Kaido's defeat.
- Kaido can't acknowledge Zoro as Oden's equal after he's defeated
- Kaido can't get closure on the emotional scar Oden gave him after he's defeated
- Zoro can't fulfill Oden's will if he's not responsible for Kaido's defeat
- Blackening a sword may have some connection to fulfilling the will of the sword
All of them are tied to Oden's unfulfilled will and Kaido's Oden trauma; they expire when Kaido is defeated.
Nah, Kaido wants to die at the hands of a monster samurai on par with Oden. When the Scabbards attacked him with Togen Totsuka he said he saw in them the visage of Kozuki Oden. That with every throb of his scar, he remembers. He could have let them kill him, but they are not Oden. A monster samurai of his like will never be seen again. Their wound was too shallow. They don't even have power to open up the old scar. They cannot do it.
It's not about recognising potential. Kaido literally wants to die at the hands of the samurai bearing Oden's mantle.
You said that Kaido got significantly stronger, and that Oden only fought him in base.
Let's say significantly stronger means Kaido is now 30% stronger.
Let's say that Hybrid Kaido is twice as strong as base Kaido.
Let's say the Kaido that fought Oden was a 9.
That means that base Kaido now is a 12, and Hybrid Kaido is a 24.
How do you go from a 9 to a 24, and keep them both within the top tier range ?
There's a huge difference, so Oden becomes way less of a relevant benchmark beyond the trauma factor that Kaido kept with him, in spite of growing that much stronger since then.
Zoro's never going to get a 1 vs 1 fight that ends with him defeating Kaido and killing him, so that we can forget about. The one defeating Kaido is Luffy, Zoro is there to help and open the scar.
Even in the case where you propose that he has to be the one who kills Kaido, it still does not come packaged with him defeating Kaido. That would be Luffy's doing, so then would Kaido not want more to die at the hands of the one who defeated him? You can say Luffy is not a swordsman, but didn't Kaido specificaly look at Luffy and see Oden among the others when talking about how far Luffy's ceiling could go ?
I think Zoro's utmost contribution will be the opening of the scar. That's what the Scabbards were unable to do, and that could be what allows Luffy to force his victory against Kaido. I don't think Kaido dies, but even in the case where he is defeated by Luffy, and then Zoro kills a defeated Kaido, is that proof of surpassing Oden ?
That would imply Oden had someone else defeat Kaido for him, but then failed to kill a defeated Kaido. And that's obviously not how it went back then. It would be more like, he's the only other Samurai that could have killed Kaido after Oden, but that could also mean he's the next best thing and only one who can overall match and eventually surpass Oden, instead of evidence that he has already surpassed him.