If Luffy beats Kaido now in a 1 v1 it would no way be considered a "Solo". The guy has taken more deadly attacks during his fights on the roof than any other character we've ever seen in the series.
While Kaido's taken a lot of attacks from a lot of different characters the only attack that seemed to have any real effect was Asura, and it's not clear to what extent. If the 1v1 begins here, I don't think it's obvious that Luffy is going into the fight with a higher health bar than Kaido. While Kaido has been taking damage from characters of varying strength, the damage that Luffy has received has come from two Yonkos.
Also within the universe of One Piece, I think it's important that Zoro shares in the credit of bringing down Kaido--which will only really happens if he lands the finishing blow over the flower capital, otherwise all credit will probably fall on Luffy.
1. Zoro needs to become a legend in his own right as Rayleigh was and accomplish something so that his name "will reach the heavens". There are not a lot of other opportunities for this as it is unlikely that Zoro will duel Mihawk on paper-view or in a public forum.
2. If Zoro is to become the WSS, it should be recognized within the OP universe, which means Zoro should have some level of renown prior to dethroning the current title holder.
3. Given Zoro's ancestry and the Ryuma/Oden parallels, it would be fitting if the citizens of Wano recognized Zoro's role in bringing down Kaido above that of Killer, Kidd, and Law.
4. Also if his bounty is to go up by a billion, the marines should have a concrete reason