Half of the Russian scientists who left the country publicly opposed the invasion of Ukraine: they posted online and signed an open letter against the war. In the past 18 months, at least 21 Russian economists, 27 computer scientists, 34 physicists and mathematicians, 15 biologists, and 17 philologists left the country.
There are a very limited numbers of scientist and even more professors at top university. One professor moving, is basically closing down a whole research branch.
One professor leaving is irrelevant. He will simply be replaced. The new professor might be better or worse at some of the many, different things he does. Not to mention in Russia… how do you get to such a position? Actually with good work?
What is relevant is when a large portion of the highly skilled population leaves.
Professors are experts on a very specific fraction of a scientific topic, even losing one as I said - might close down an entire research branch. And as you said - Russia already has a problem with corruption, so not many scientist have to leave to shut down what is left of research.
That’s not how that works. Professors at large aren’t fungible. You cannot replace one for another.