Don't underestimate the ingenuity of airlines under an embargo . As said earlier Iran managed successfully since over 40 years to buy and get a fleet of Airbus, Boeing , ATRs and Fokker flying ,, mainly due to the use of third counties sympathetic to Iran providing the aircraft , the spares, the software updates etc.. . Russia is a bit more isolated at the moment but if for instance China supports them , they can continue to fly for a long time, and when this war is over and sanctions lifted , they can , just like before use their Soviet and recent Russian made aircraft to fly internationally.
The other plausible scenario is of course a regime change in Russia, the past is forgotten , and the West happily sells them new western aircraft ., just like in 1990 ...