Demographic decline can leave water distribution systems (WDSs) oversized for the demand they now serve, increasing maintenance burdens and, under reduced flows, the risk of long water residence times. This paper examines the contraction of existing WDSs under sustained demand decline. Pipes are represented by separate physical-retention and hydraulic-activity variables, allowing reversible isolation without immediate asset removal. Reliability is protected through node-specific vertex-connectivity requirements, while candidate pipes are ranked using hydraulic utilisation and edge betweenness. Each proposed isolation is checked against hydraulic constraints, including a water-age guard that prevents new unacceptable conditions or further deterioration at nodes already exceeding the adopted limit. The approach is tested on two networks with contrasting topology: the densely looped Parete network in Italy and the more branched ky3 benchmark network from the United States. Approximately 29-32% of pipe length can be isolated in Parete, compared with 16-18% in ky3. Greater demand decline does not necessarily allow more of the network to be isolated, showing that feasible contraction depends on both topology and the hydraulic and reliability constraints that become limiting as demand falls. Water-age response also differs between the two networks, remaining low in Parete but becoming increasingly restrictive in ky3. Overall, the results support reversible pipe isolation as a practical adaptation strategy for oversized networks, contributing to long-term water security without immediate permanent decommissioning.