Kiev can be used as a threat in order to get the Ukrainians into the negotiating table. Do the occasional shelling but don't attack it yet
Ukrainian will fight for a long time under NATO encouragement unless it is deprived of the ability to fight, or it suffers such mass casualties that those not eager to die for ukraine will press for ceasefire because their internal anslysis pegs their own chances of dying at too great a level.
Leaving Kiev as well as most of the country unoccupied is a recipe for indefinite extension of the war by leaving ukraine with both the ability to fight and the room to have that capability sustained by nato, and make it tangible that many ukraians can press for defence of ukraine without being personally much endangered.
Not taking kiev quickly is a strategic setback for russia comparable in effect to hitler’s decision in mid to late summer 1941 to procrastinate till october before driving on Moscow.
it may not be immediately fatal to the entire war effort by itself, but it constricts later options so that odds facing later actions become steeper and a later failure would have much greater negative repercussions. So if the entire effort ultimately fails, one could chase the chain of failure back in time to this event.