Well, here is my theory. In my opinion, Urashiki Otsutsuki will not die during this Momoshiki Otsutsuki Arc. From the conversation that he had with Momoshiki about Momoshiki always underestimating him and all. I think that Urashiki Otsutsuki holds some sort of grudge against Momoshiki Otsutsuki.
And when Momoshiki Otsutsuki will be battling the five Kage, Sasuke and Boruto Uzumaki, Urashiki Otsutsuki will just observe from afar and will have his own motives. That’s right I think that his motives are different from that of Momoshiki Otsutsuki’s. I also think that when Momoshiki Otsutsuki will be about to be defeated and Killed, Urashiki Otsutsuki will not come to his aid and will either try to fulfil his own plan or will just flee after seeing Momoshiki Otsutsuki getting defeated.