This is probably incomplete information, but wine grapes aren't commercially grown from seed as much as they are grafted onto root stock with whatever varietal is desired for that vineyard section. My guess is that there are already phone calls going out to vineyards in France (where California transplants saved French wineries years ago), South America, and Australia to get new rootstock and grafts. The wine will be back to 'normal' in a few seasons. Many heroic recovery stories are already being envisioned by the winery owners. Insurance companies are going to raise rates on agricultural fire insurance, and more non-wine fields will be converted or proced out of farming in California. It's hard to be a pig farmer when growing grapes is so much more profitable.
One thing I always disliked about California wines was the pricing. They charged what they thought they could get for a bottle, vice what it made sense to charge based on quality or other market factors. An otherwise twenty dollar bottle of wine would be sold for fifty or more dollars just because it was a fancy brand in the right county. Of course this is capitalism (as practiced by anticapitalists in many cases) so if they can do it, let them. I'm sure pleas for bailouts will come before the end of the year, with small farmer and non wine growers lining up far behind the politically connected and bigger grower operations.
Now that you mention it, you’re probably correct that even if the rootstock survives, the varietal “upper” grafted to it may very well be dead and whatever resprouts will be the rootstock variety.
Buyers beware.
“It’s hard to be a pig farmer when growing grapes is so much more profitable.”
And safer. If you have a heart attack in a vineyard, the grapes won’t EAT you! :O(