Adding on, the thing about Yahiko that quite a few of us seem to agree on is that with him being alive, his ideals don’t really fit with the Akatsuki’s goals. Yahiko never planned on gathering the tailed beasts, let alone creating an ultimate weapon to nuke the world in the name of peace.
As for Nagato, all his ideals came from Yahiko. When Tobi showed up, Yahiko firmly denied his ideas and told Nagato and Konan to stay away from him. It was only when Yahiko died that Nagato joined Tobi to gather the tailed beasts and create the ultimate weapon.
With Tobi currently out of the picture and Yahiko still alive, it just seems really unreasonable that Nagato is willing to go to this extent. At the very least, Yahiko should definitely be against this whole idea. The only problem with that is Nagato’s ideals come from Yahiko and he would more than likely listen to him. Would be an interesting conflict.
At around 3:00, you can also see Nagato about to lose his mind and kill nearly everyone only to be stopped. I interpret this as Yahiko remaining alive virtually prevents Nagato from going crazy. If anything, should Yahiko remain alive, there needs to be a lot more convincing as to why his ideals will have shifted so drastically.