The New York Subway closes parts of lines every few years for heavy maintenance and inspections - by closing those lines at 10PM and reopening at 5AM instead of running 24 hours.Although New York City's system as a whole is a 24 hour service, not every route operates at all times. Portions of the system do shut down on a nightly basis, and portions of the system are similarly closed on weekends and holidays. Additionally, weekend service changes for maintenance currently happen in New York City, equivalent to current WMATA operating practices.
There is no model by which Metro could start shutting services, say, at the end of evening rush hour and manage to complete major repair work substantially faster than under the current model of weekend closures. There's a substantial time cost to setting up heavy repair equipment and maintenance teams, and the real time savings come from being able to stick the equipment in place and keep work going until it is finished.
Future intensive repair work might look more like Chicago's model, where a part of an entire line simply closes for a few months with buses to replace it.