At this point, this is the only way how Oda could hype Kaido back again...like this was litteraly game of BigBitch herself, so in the end it also could mean that Kaido would't be defeated without BM defeated.
Plus this will be good for BM portareyal case that would made her real mastermind, who could anytime use Kaido in her plans. That also explaine why she was so confident that Kaido will agree on alliance with her, case he can't kill her, even if he say that he could.
But lets see...
Oda talk in manga that Kaido head would chop off to many times...so that indeed could happen, but just to reveal, that Kaido is immortal.
I was always on this theory, from the moment when BM told Kaido about his debt to her, and the fact that this is not just long time ago debt, but Long-life debt.