Yamato fought all that time and couldn't even damage Kaido substantially.
Only her thunder bagua was enough to draw some blood from Kaido but he was still unphased, now compare that to how he reacted to Asura.
Zoro will always be treated as Luffy's vice captain and lately the whole "wings of the PK" theme has been hammered home. Even if Yamato joins, Zoro and Sanji will be treated as Luffy's right and left hand, power levels don't really matter much in the end.