The issue is that people are moving into land that wasn't used for irrigated farm land. If we in-fill to 2x the population density, there isn't any farmland to de-water. And most farmland in the southwest is just grazing land that isn't irrigated.
There's a surprising amount of farmland in the bosque, which runs alongside the Rio Grande for most of its duration, and a lot of that farmland is low-value crops like alfalfa. I would guess this is due to a combination of not wanting to pay for it for their cows and not wanting to fail to use water rights, lest others try to make claims.