Black Zetsu has been able to stay alive for so long since he's been hiding for all this time. Now he is placed himself in front of everyone, he cannot hide anymore. (Meaning he would be killed unless he found some way to overpower them. Like Kaguya for example.) Black Zetsu would need to steal the Rinnegan, wait for Madara' to be unsealed, then find another Uchiha,Senju or Uzumaki to use the Rinnegan and throw away their life in order to resurrect Madara.
Yeah... That would have taken decades at best. Not to mention that the only 3 people who can preform Edo Tensei are Kabuto,Orochimaru and Tobirama. None who would revive Madara a second time after stopping him once.
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Even if they didn't do it, there is no saying that somewhere down the line somebody else will come along and do it again as the jutsu can be learned by others, or BZ could manipulate someone else and teach them how to do it after stealing a scroll containing the info on how to do it.
It would take decades yes or maybe centuries, but it is still possible as long as BZ and the Rinnegan exists to do this again.
As for BZ's survival, he escaped death when Chojuro tried to kill him and didn't show up until after they all went after Madara and decided to let Obito live.
If Sasuke had killed Obito, then BZ simply wouldn't have shown up and nobody would have been the wiser.
As far as everyone knows he is just a mindless drown for Madara, with him dealt with they wouldn't have bothered looking for him.