The capitol is symbolic, but also where the heads of state have fortified thier assets. Russia's goal is to eliminate "foreign" controlled assets. They could in theory surround the capitol and isolate it, which would neutralize those assets, then set up thier capitol in another city.
But that would look incredibly weak and would be far more costly than razing the city to the ground. And having the old gov hold up anywhere in the country would be like having a well connected terrorist cell in the heart of your country.
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