As Russian Deputy Prime Minister Dmitry Rogozin pointed out last year, "The Russian segment [of the ISS] can exist independently from the American one, but the American segment cannot exist without the Russian."He's full of crap.
I agree, mostly.
The ISS was built around the Russian modules (Zarya, etc). Theoretically they can function without the rest of the ISS (they did, initially, being the first pieces hauled up) but a big chunk of the ISS would have to be disassembled in order to get them out.
THEN they’d have to be shifted to a completely different orbit. Which isn’t going to be cheap or easy.
My guess is that this is all pre negotiation posturing/groundwork laying by Russia to either get paid more for it’s involvement in ISS (with there now being commercial competators to Progress for cargo and soon to be competators for human transport) or to have the other nations buy out it’s portion. The latter probably with a lucrative long-term contract to provide technical support for it’s old modules.