Roscosmos is pretty ****ed right now, and NASA should be planning on moving forward without Zvezda. Russia has threatened to leave the ISS and decouple Zvezda before. ISS doesn't have it's own propulsion system and uses Zvezda. Apparently station-keeping can also be done via a docked Soyuz' propulsion. Hopefully Crew Dragon and Starliner have been designed to provide propulsion as well.