Of course a trial court's erroneous decision holding that a party isn't entitled to a jury trial should be reviewable (in an appropriate case) by an extraordinary writ. We shouldn't force the parties to waste money (or allow the losing party to sandbag) by making them participate in a meaningless trial.
Kudos to the California Supreme Court for unanimously overruling its prior precedent to the contrary. It's good for the law to be clear (and right).
If only so lower courts won't feel compelled by precedent to do the wrong thing.