The most likely outcome for the yucca plant, if the yucca moth goes extinct, is:
The yucca plant would also go extinct.
This is because the yucca plant relies on the yucca moth for pollination, and without this specific pollinator, the plant would likely be unable to reproduce effectively. While other options like engineering human intervention or adaptation of other animals could be theoretically possible, those scenarios are not guaranteed and would likely take a significant amount of time.