Well, no. They would be cut off from other countries/ISPs that drop the BGP routes. But Russia could just allocate IPs as they see fit inside their network and

Well, no. They would be cut off from other countries/ISPs that drop the BGP routes. But Russia could just allocate IPs as they see fit inside their network and coordinate with friendly countries on routing arrangements. Although things would obviously get difficult for third parties if the IPs are assigned to other entities. It would be a huge disruption though.