Oonoki, Gaara, and Mei all needed to be healed and needed their chakra replenished.
So firstly how would Jiraiya have gotten to the battlefield?
Then with Gaara and Oonoki on the brink of death who would have stalled for time to prepare sage mode?
Mei would either be dead or still fighting white Zetsu. Jiraiya would have no way to get there. Or die in the process of trying to get there
Gaara and Oonoki would be pretty easily killed.
A is not capable of stalling long enough for Jiraiya.
Jiraiya dies.