Energia, the division in Roscosmos that manages the Russian manned program, including its share of ISS, has proposed that Russia build its own space station after 2025, and the reasons it gives strongly suggest they are worried about the condition of their 20-year-old and leaking Zvezda module.
The International Space Station’s systems are likely to keep breaking down after 2025, Energia company, which manages the Russian module, said and suggested building a national station instead. “Russia has commitments regarding the ISS through 2025. Several elements are already seriously damaged and about to stop functioning. Many of them are impossible to replace. After 2025, we expect many ISS elements to start breaking down en masse,” the deputy director-general of Energia, Vladimir Soloviev, said at a meeting of the Russian Academy of Sciences. [emphasis mine]
They don’t mention Zvezda by name, but the highlighted words point to it. Assuming the persistent leaks in the module are related to stress damage and age, this statement is recognizing publicly that replacing Zvezda is very difficult, or impossible, and that it is on the verge of complete failure.
Whether Russia has the money or resources to get another station built soon after 2025 is questionable, however. If they do decide to go it alone, I doubt they could launch a replacement before 2030, at the earliest, and likely much later than that.
If they do end the partnership with the U.S., it is unclear what happens to ISS. If the Russians choose to undock their portion of the station it will leave ISS somewhat crippled. It has been my impression that without all of the environmental systems in the Russian half, ISS cannot function. This might no longer be true, and will likely not be true with the planned arrival of Axiom’s private modules by the mid-20s. Timing however of all this remains critical.