You still control the access point. So it means the Russians control it, but they can't break out of it. Russia couldn't take Istanbul, the worry would really be Russians power in the Balkans, turning Bulgaria and Romania into a alliance against Turkey. The US did attack and partition them, and they recovered over the next 30 years. The Germans attacked and partitioned them, they recovered 20 years later.
How is Russian imperialist? It hasn't invaded anyone, just claimed people who want to be part of Russia and that's it. Its like the UK with Gibraltar and Northern Ireland. You Turkey with Cyprus. Turkish is the rising power here, you demographics are excellent, you have stronger allies around you, you have weak nations around you. Only after you have made them bend the knee could to move onto the big bear and carve it up. So don't be so worried about Russia, it is a declining power in its last throws trying to salvage what it can and hunker down to survive. It has no proper alliances which it isn't having to directly support.
So the Russians are greatly over hyped as a threat to anyone. So is China. I view Turkey and Japan and India and France as the rising powers in Eur-Asia. Which is why I want Britain to have nothing to do with Eur-Asia for the next 100 years or so until it dies down a bit.