I mean right now the alliance have no one chance of defeating Kaido and BM with just their strength......they need a very big help
They dont even Need to beat Kaido AND BM, it will be enough to beat Kaido.
Also, there is some improvement still coming before this final fight, at the Harbor when they do the first fight, which will be separated from Onigashima-Fight. There is a way to improve even more, than we have Luffy already directly prepare for Kaido and for that, he is optimistic to go for Kaido, even when he know how strong Kaido is, saw him in his Dragonform and got attacked by him. Luffy is a Retard most of times, but dont forgett, like Goku, he is a Genius in fighting and battles. If he knows the strength of someone and still wants to go for him, he is able to do it.
When we are talking about Big mum additionally, then for sure, right now we got some Problem, because a direct fight seems difficult, but there are certain ways to get rid of her. There is tama, there is a lot of People still left for fighting, the heck, even the whole Mink tribe could go for Big mum in Zulong form.