They won't have to. In 10-20 years, I'd wager the area would be mosty populated by Han Chinese (which Russia is actively encouraging due to their lack population in the area) and they'll do a Russia and setup fake referendums with their intention to join China and Russia won't be able to do a damn thing considering they'll probably not even exist in their current form.
This is a realistic take, not sure about the fake referendums but those areas will definitely be populated by mostly ethnic Chinese. Why even bother invading Russia when you can just emigrate there legally and turn it into China over time? A declining Russia won't have the administrative power to hold together such a large country.