I do think it would end any Russian politician's career if they were to tell the nation that the war has been in vain and that the occupied territories of Ukraine are to be returned but I also think you underestimate the power of a dictator. Everything in Russia is built around Putin and his image, so him dying could be a significant disruption leading to another revolution. That said, in Russia, virtually every revolution leads to a brief moment of freedom only for another dictator to come in and say how chaotic it is and that order must be restored, with the people accepting that.