Depends on what your time frame is, if it´s just Wa No, sure, but it´s not a big enough to see it as rule, especially since Big Mom is not a F6 or commander, meaning important enough to have a different conclusion.
And outside of Wa No, there have been vs titles without conclusion, or someone else beats one of the participants, and so forth, whether it´s dog vs cat, luffy vs sanji, sabo vs Fuji, and so forth. Of course these are not within the context of the main fights of an arc, but past examples also always only included the Strawhats in the arc vs chapters, and whether it applies to these guys as well, we will see, for me at least not conclusive.
1002 was Yonkou vs New Generation, technically that fight is still on-going, or at least went on until Luffy fell down from Onigashima.
Personally, i wish she would get defeated, i am over Big Mom long ago, just don´t see that it has to be happen from the current setup, and arguing Law and Kid would be treated like Strawhats does not work since at least Dressrosa in which Law had all the arguments on his side to beat Doflamingo considering the baggage, yet turned into a cheerleader for Luffy.