That's no defense.
And the difference between digging in your tanks and infantry in Ukraine is what?
When Russia moves in on Ukraine proper—not Crimea, it will be based on protecting the Russian population in Eastern Ukraine, and the default of Ukrainian loans ($20B plus or so) to Moscow.
Moscow can call those loans due at just about any time.
The combination of the two will “justify” the move.
My guess is that will deem the military assets to be collateral for that “collection.”
Eastern Ukraine is gone.
Color it in on your map.
Everything that borders the Azov sea is gone.