The U.S. has 10x the defense budget of Russia. More than that, it has the best brains from every nation on Earth who emmigrate to Western nations.

Since WWII most nations have decided to ally themselves with the U.S. or Russia. Ukraine wants to be part of the U.S./EU alliance and are literally dying to get that memberhip card punched.

Russians don't want to fight (those at the front) and Ukrainians won't let them win and have the means to decimate. Therefore the simple calculus is which side prevails, the one with 10x the military or the lesser? The gap is just too wide. Now, the U.S. may eventually lose to China, but Russia, most people don't see it.

If you can argue how a military 1/10th the size of the other will win please do it! As for the U.S. losing interest. Until that happens I don't believe it should be in the calculation.