Replace Putin with what, if that's your point?
I don't think Putin is getting replaced. At least unless he actually does this:
I believe that they will go fully into a nationwide mobilisation, and as such the war will go on for years.
Full nationwide mobilisation is going to be even more of a problem for Russia, the setup needed for it was dismantled after the fall of the Soviet Union. They don't have the training facilities any more, and creating a large body of armed, angry people who don't want to go and pointlessly die is exactly how you get a revolution (see the thread from Kamil Galeev linked above).
I dunno, I think the most likely outcome is a war that rumbles on in the East for years, but I don't think Russia is going to go mass mobilisation to support it. Instead, I think they will dig in and try and keep the captured territory, link Crimea with the Donetsk and Luhansk, and keep fighting until Ukraine accepts the loss of territory in return for a ceasefire. However, I think a climbdown is also plausible, abandon the expansion of territory, hold on to Crimea, Donetsk, and Luhansk, and claim the "Special Operation" achieved its aims.