#3 To turn on her again

Becky Lynch may be a fan favourite now but don't forget that she's still technically a heel. Just because she hugged Charlotte Flair doesn't necessarily mean that it's all well and good.
It's true that they likely won't be feuding for a while to come since their rivalry concluded at Evolution, but even then, if Charlotte beats Rousey, it gives Becky Lynch a reason to turn on her for taking her spot. If she loses, it also gives Becky Lynch a reason to mock her further.
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Either way, Charlotte Flair may not necessarily get a hero's welcome from the champion after Survivor Series. But with that being said, it does seem logical to keep the two separate for a while now. The feud they had after the summer of this year was magical and it gave Becky Lynch a much-needed boost after two years of ups and downs (mostly downs).