While they saw the value of the worm in economic and spiritual (maybe even political) aspects, they wanted to achieve a balance where the ecosystem would suppor

While they saw the value of the worm in economic and spiritual (maybe even political) aspects, they wanted to achieve a balance where the ecosystem would support both species. They trapped water underground because that was effective storage against evaporation, and to keep it secret from the ruling Houses. Spoilers below: Down the line, Arrakis does get terraformed and a vast planetary desert preserved for the continued existence of the spice cycle.