Oden’s final act was so impressive that it inspired Kaido’s own son to idolize Kozuki Oden and live the next 20 years as another reminder of Kaido’s tainted victory.
Even when Kaido has Oden’s own son dead to rights he can’t move forward until Momonosuke throws away his association to Oden, a truly petty request. No matter what Kaido has done he was not able to truly purge Wano of Kozuki Oden and now 20 years later the reckoning has arrived.
The Red Scabbards are here to carry out their lord’s will and have each privately trained their Ryuo and learned their master’s technique enough to pierce Kaido’s indestructible skin.
Oden warned Kaido to not underestimate his samurai and to build his strength, but Kaido was sure that the will of Oden would die with him 20 years ago. Now we see Kaido in the same vulnerable position as he was 20 years ago and we get to see how he will respond to it in the present.
As nice as it would be for Kaido to maintain his invincible status until Luffy reaches him I don’t think that is the story Oda has been telling in Act 3. This fight with the Red Scabbards is the culmination of not only 20 years in their time but also the longest flashback in One Piece history featuring Oden, Kaido and the Scabbards. This is also the culmination of Kaido’s story arc with Oden and I would much rather prefer Kaido to be a much more fully realized character by the time Luffy reaches him than him remaining a generic indestructible monster.