Not at all. It's a marginal value question. When water is plentiful, people will find lots of uses for it beyond drinking it. For example, you can cook pasta with water - at the end of the cooking you just throw the water away. Water has been so inexpensive it makes sense to pour it right on to the ground to make plants grow.
Now, water is getting a little harder to come by. It still makes sense to cook pasta with it, because very little is used. But perhaps it's not so useful to pour it on the ground for every type of plant. Perhaps there are certain strains or varieties that aren't quite so water intensive. I know this is true for tomatoes.
So essentially you give up on the very water intensive kinds of plants, let the competition grow those where there is more water.
That's a simplification, some things, like orchards take years and years to become productive. In those cases, perhaps it makes sense to pipe in water, desalinate ocean water, or whatever. Competition isn't going to pick up the slack, because it takes years and years to get productive.
Food needs to be inexpensive, and readily available, subsidies absolutely make sense to make food inexpensive. But perhaps we don't need to subsidize quite as much of that 20% beyond the food we need.