From 1991 to 2021, the author notes, the number of tanks in Russia decreased from 51,500 units to 2,840 units (18 times). In the United States, the number of tanks dropped from 15,440 units to 2,509 units. Why did the state not use the old models of tanks further?
The point here is not the shelf life. Tanks are generally fairly easy to maintain, and can last a hundred years if their subsystems are changed. This could be done with the T-55 (for the Russian army) or the M60A3 (for the US army). However, the United States abandoned this model altogether, and the Russian Federation is significantly reducing the number of T-55 tanks.
What, then, is the reason for getting rid of old tanks? There are several factors contributing to this.
After the end of the Cold War, the US needed a large number of tanks. It was also necessary that the American military forces could be quickly returned if necessary. Therefore, it was required that military equipment was easy to move: for this, many infantry units exchanged armored vehicles for Humvees. This also helped to reduce the amount of fuel needed to move.