In the thirty years since the USSR dissolved no one has so much as bothered Russia or even pretended like they wanted to bother Russia. Most NATO members in Europe vastly reduced their militaries and defense spending. Everyone was content to just buy shit from them as business relationships are far cheaper than invasions and annexation. NATO only expanded east because of Russia's posturing about reclaiming former SSRs.
The only people worried about "defensible borders" are old Soviet hardliners pining for some past glory that didn't exist. They either see or pretend to see the specter of foreign invasion because it whips of nationalistic fervor. They use it to distract from their shit show governance and to blame "the West" for every self-inflicted problem they have.
The most baffling part is literally no one wants to invade Russia. Outside of their propaganda machine no Westerner has any interest in Russian territory. No one is super excited to occupy Siberia or fish the "waters" of the Aral Sea.