Big mom is kaido’s ally
if she isn’t defeated she has no purpose in wano
That's not true, she could steal Kaido's Road Poneglyph and kidnap Robin.
Just because Kaido and Big Mom are allies doesn't mean that there isn't a possibility of Big Mom betraying Kaido.
This arc also had a lot of talk about betrayal.
You have to accept the fact that there was always that possibility that Big Mom might not get defeated during this arc.
Now I personally think she will get defeated during this arc, but knowing Oda you may never know what could happen.