You cannot keep the army in there for ever. Those soldiers are costly to maintain and it seems like the Ukrainian population are not quite getting the grasp of the "liberation" and are likely to setup a long an difficult urban warfare followed by an obstinate resistance.
My concern now is that Putin cannot back from his personal gamble and if this go guerrilla he may commit atrocities beyond what we think possible - going Stalin mode on Ukraine.
In my view, Ukraine cannot be defeated until the Ukrainians themselves decide that they have been defeated.
Kiev could fall if NATO haven't deployed its special troops to the capital. I know officially NATO is not involved but there are plenty of proofs that Black Ops unite are being deployed in Ukraine. Now its Russia vs NATO unofficially. Most of the Russia's main force are still not deployed and their advanced tank and aircraft still on the ground. I do not know what Putin's ultimate objective is but after so much preparation it seems he is not interested for a quick victory which concern me most. I think his objective is something else that's why he mostly using mercenary unit and keep his main force in reserve. Pretty sure he hasn't achieve his first object yet which is still unknow.