I am sure plenty of expert analysts have been working on scenarios of what would happen if Putin was toppled or/and if Russia was totally humiliated in Ukraine. Maybe those in charge do not like the most likely ones.
The question isn't whether Putin will leave power someday: He will, that is indeed a fact. The question is what would happened if he did now, especially if he were to be toppled. If the US reckon that he outcome would not be good for them compared to the current situation then they won't push for it but rather wait and/or maneuver to change the conditions so that the likelihood of a better outcome increases.