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 specifically on the yucca moth for pollination, and without the moth, it would struggle to reproduce effectively. While it's theoretically possible for other options to happen (such as humans intervening, new animals evolving, or the plant evolving a new pollination method), these scenarios are less likely given the specificity of the mutualism.