Another city would be picked to represent the rest of California, and the point is that using that city as a time zone ID would then work not only for future times, but also backwards for any times in the past. If, instead, you had America/California, or US/Pacific, those would be ambiguous either forward or backwards in time.

So the trade-off is timezones being specific to a particular city but remaining unambiguous forward and backward in time.

You can’t avoid pain when there is a change in the geographic area of a legislative time zone. But you can avoid the case of a time zone ID becoming ambiguous in terms of the UTC<—>local time mapping it’s supposed to define. The latter is the aim of the present scheme.