According to The Wrestling Observer Newsletter, stories circulated that Charlotte Flair had been promised that the main event was incorrect. The original plan not even for Rousey was the main event. The original plan was for Roman Government to make headlines, before the diagnosis of leukemia, possibly Dean Ambrose.
Rousey vs Natalya will be the main event for women on the Raw side, but the two things that put it out are that Lynch and Flair are injured, and that Jim Neidhart died, and once Neidhart dies, Vince doesn't do it. I don't want to change Natalya to Rousey. Rousey vs Flair has long been expected to be a match and even a fun event but everything has changed. Then Lynch gets hot and almost everyone is on the same page for Rousey vs. Lynch at Mania and Rousey vs Flair at Rumble.
Rousey vs Lynch is a Mania match. The decision whether it will be the main event and last last has not been made, and may not be made so close to the time of the show; it also can still end in triple threat with Charlotte.