In a nutshell, the answer to the question is that Russia has already lost many battles to Ukraine and is basically humiliated that it has not been able to overrun Ukraine with ease. But Russia may very well succeed in holding much of the east and south of Ukraine. But is that what really matters to Russia? Not necessarily.
What truly matters more than whether or not Russia takes over much of the Ukraine is whether or not Russia is able to maintain it’s own population from rebelling against it’s leadership. As much as Vladimir Putin would like to believe that he is bringing back the power of Russia to be a dominant power on the world stage, the fact of the matter is that Russia is an aging country who’s population is not looking to restore Russian power as much as they are looking to modernize and live a better life than their parent’s life.
Vladimir Putin is a dangerous and powerful person who will probably lose his base following his fateful decision to draft hundreds of thousands of Russian soldiers. The main job of the Western NATO countries should be to destabilize Vladimir Putin without violence. The key to that is by continuing to broadcast messages to the Russian population via the internet that offers them incentives for leaving Russia.
Although leaving Russia may be illegal for army-age men today, it is not illegal for others. And those who don’t want to serve know about the holes in the Russian border. Putin must be destabilized, not just to protect Ukraine. He must be destabilized and weakened in order to make sure that Russia and China discontinue their strategic cooperation that threatens the free world.