What makes Yuri’s Revenge a "forever game" is the sheer variety in unit design. It introduced iconic tools that changed the meta:
Why build a tank when you can just take your opponent’s? Yuri’s Clones and Master Minds made every enemy unit a potential traitor. Command & Conquer: Red Alert 2 - Yuri's Revenge
A literal rolling fortress that infantry could shoot out of. What makes Yuri’s Revenge a "forever game" is
The biggest draw was the addition of Yuri as a standalone faction. Moving away from traditional tanks and paratroopers, Yuri’s arsenal focused on and genetic mutation . Command & Conquer: Red Alert 2 - Yuri's Revenge