He "died" because of Kaido.
Yes, and the effects of that were still there when he came back.
Which is my point, essentially. If Kaido was weakened from prior fights, Luffy was weakened from Kaido beating him deader than dirt.
I've seen people suggest that they could have but didn't because of "balance" except they picked a fight with Whitebeard. I don't think the Marines would have backed down from fighting Shanks at Marineford if they didn't have substantial loss from that, either.
My understanding of MF was that it was never about WB, it was about Ace.
They wanted him publically executed after being revealed as Roger's son to demoralise the up and coming pirates as well as take care of a threat.
WB was never the main priority.
As for Shanks, Sengoku explicitly says "since it's you, red hair" that he'll stop the war. Again I guess it's a matter of whether they want to fight Shanks or no, and if this guy is a CD descendent with importance to world balance, that's a good reason not to fight.
The Marines themselves were still in good condition, hell they'd been stomping so hard up to that point that Koby lost his marbles and started telling them to chill.
But he already clashed with Kaido. Oden aside, the chance to take out Kaido presented itself. You can say he was wise enough to save the trouble because he was definitely needed at Marineford, but my point is just that beating Kaido wasn't something he could just do.
Again it's a matter of whether he wants to.
Why exactly would Shanks want to kill Kaido aside from maybe the Oden thing? We also have no clue about their interactions either. Shanks' motivations also are a mystery atp.
And outside of their MF interaction, he couldn't exactly take on Kaido in a war. The latter seemingly had more potent armies and a stronghold in Wano.