I would prune as close to the main trunk as possible, and check the cross-section to look at the vascular system.

Occasionally, the main trunk on California sages splits along the division between the four corner vascular bundles, and each of these in effect can become separate plants with their own root systems. I would not count on this, since a microbial attack (bacteria, fungus, or virus) could easily doom the whole plant.

Agreed on the pruning all the way to the trunk, CA sages can give up the ghost quickly if left to die back, but they will also regenerate after heavy pruning. A fire adaptation strategy, I'd guess. LA temps are jumping to 100 this week, hot and moist soil is the recipe for root rot, so I'd suggest mulching instead to conserve whatever moisture is still there and then watering when the fog rolls back in. People seem confused about wheter this sage is purple sage crosssed with S. clevelandii or S. sonomensis, two sages with completely different watering needs, so I'm not sure what its needs are. If you do replace, perhaps look for an heirloom CA sage over the hybrids, Salvia brandegeei grows only on the channel islands and it has an amazing scent & draws bees, it is very well adapted for LA.