No-one has interest in Russia losing in a way that would cause instability within Russia up to possibly a civil war. No-one wants to see the return of the chaos of the 90s, including risk of nuclear dissemination.
The continuation of the war also suits interests, especially for the US, broadly by ensuring a continuing split between Russia and Europe and by creating headaches for China, as well as, of course, keeping Russia isolated and weakened.
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.
The continuation of the war also suits interests, especially for the US, broadly by ensuring a continuing split between Russia and Europe and by creating headaches for China, as well as, of course, keeping Russia isolated and weakened.