It is sort of a trope in CA, oh and it's the deltas, are you saying that stinking up their bathrooms with un-flushed urine and being charged a surcharge to save 10% of 10% of the available water is logical?
Residents weren't being asked to save 10% of 10% because taking that water from the environment would destroy it. They were being asked to save 10% of 10% so agriculture couldn't point to residents and say "see, they're doing it, we should be able to take whatever we need to from the delta too".
No one is arguing about wet years either. Look at the dry year example. 36% environmental. 62% agriculture. 13% urban.
Edit: also notice the chart - the delta is only about half of those environmental percentages (on average, unfortunately they don't split it out for dry years). The other half comes from "North Coast" which seems likely to be more difficult to take advantage of - at least I haven't heard any controversy over trying to use that water. We don't know what the exact percentage would be in a dry year, but it would still be below 36%.
Australian toilets frequently have two buttons, one for a small flush (for urine) and another for a larger flush. "No flushing" is hardly the only option.
California needs to tackle water usage. There are reasonable changes that can be made in homes and towns - swapping to drought-resistant gardens instead of lawns, rain barrels to catch run-off, absorbent parking lots that don't drain out to the sewers, etc.
Agriculture also needs to be tackled, sure. That doesn't mean saving a percent here and there is of no use.
This style of toilet is already very common in California, as is drought resistant landscaping. There could be more, but California could already be past 50% towards best case on those approaches.
I don't think it makes sense to look at the total supply of water and use that as a starting point of the savings. The total supply of water is only useful in the context of a discussion of multi-billion dollar infrastructure improvements to expand the ability to transport water from one place to the other.
What really matters is the amount of water that can be channeled to your place of living today.
And then having small changes like non-flushed toilets help increase awareness about water shortage. If that ultimately makes people replace their lawn with desert plants, I think it's a win.
That's a funny way of saying "we didn't want to dry up the rivers", isn't it?