Cultivation may be splintered if Odium returns to Roshar, but I don't think it will be a simple matter. After all, why didn't Odium splinter Cultivation before/when/after he splintered Honor. In fact, why did Odium ever leave Roshar in the first place especially after splintering Honor? Assuming the the "Three of Sixteen ruled" epigraph refers to Honor, Cultivation, and Odium ruling over Roshar, then there was likely some sort of balance or agreement between the three. It may have to do with how the shardholders interpret their Intent, but I believe that the relationship between the three is more complex. A fun, little theory I have is that Tanavast and Rayse were brothers squabbling over a love for Cultivation’s shardholder. Out of rage Rayse killed Tanavast who was blind to the possibility of his brother killing him due to Honor’s Intent, much in the same way Dalinar was honor-blind to Sadeas’ betrayal. This would also explain Cultivation’s survival if Rayse cared for her. Regardless, the point is that the relationship between Cultivation and Odium is complex, and her shattering won’t be as simply as Odium showing up and immediately destroying her. Whatever held the balance between the two will have to be destroyed or circumnavigated.