I don't recall if it was ever stated or not. What has happened with their crews/territory? When Whitebeard died it was said that Blackbeard started taking his old territory almost immediately.
Kaido is probably dead I'd wager, we actually saw him getting submerged in the magma before it erupts. Plus I'm not sure what he would add to the story if he were to be back.
Big Mom is guarenteed to be alive. She outright said this wouldn't kill her after falling down that hole, Pudding iirc was convinced she wasn't dead, and she has hanging plot threads that could be explored with her return.
I think they are. If not even worthless characters die most times, I don't think Oda would kill them. I bet Vergo and Monet are still alive as well lol. Idk with Monet since she had her heart pierced but who knows... That Pell from Arabasta exploded with the bomb and survived. Maybe Pedro is alive and will make a final return as well haha. Though I think the way they set that up... Pedro is dead. Monet likely too. Not Vergo imo.
I think there would be a different build up if the Yonkos died, specially Big Mom who swore revenge iirc. Though I don't think they'll have an impact anymore otherwise it'd top the scales too much since they're way too strong.