Russians can buy into BTC & keep their BTC.
Why would they have to cash out just before any Russian ban on BTC?
How would the Russian government know if they had a stash of BTC?

They could quite easily have a bank account in Europe or the US and if they ever need to cash in their BTC they could sell them for fiat & have that fiat in a foreign bank.

Russia can put a BTC ban in place but they can't stop people from using it.
Drugs are banned in most countries, people still do it.
Speeding is a crime, people still do it.

A Russian ban on BTC may be a deterrent but they can't physically stop people buying BTC.