Russia has already moved on to "Plan C." They're conducting indiscriminate shelling against residential neighborhoods [1]. Kyiv could be the next Aleppo or Grozny.
I don’t agree with the premise that Putin can’t win the war. With enough brutality, he will subjugate Ukraine, and from his past actions we have to assume that he’s capable and willing to see this through, whatever the costs. The apparatus of siloviki is ideologically aligned and will continue to back Putin. A coup d'état is very unlikely. At this stage, Russia is "all in" and will do whatever is necessary to put Ukraine under its control. NATO won’t directly intervene unless they are attacked (and then all bets are off). It’s difficult to imagine how Putin could not achieve his goals with regard to Ukraine.
Putin has already failed on most if not all of his goals in invading Ukraine. While decapitating the government is still possible, that is only one step in the process of subjugating Ukraine.
There may still be a lot of optimism one week in, but the question is will there still be that much optimism 4 weeks in? 8 weeks? Russia is all in -- they have plenty of time and manpower to throw at this
[1] https://www.aljazeera.com/gallery/2022/3/3/photos-shelling-c...