Why do people want constantly new games otherwise they think that something is dead? This is like saying basketball is dead because basketball people only play basketball and not something else.
Because in the time between Warcraft I (not the first RTS but let's take it) and Starcraft, each new game was amazing compared to the previous. People who grew up in that loved that; but we've kind of leveled off, you can't really make things as amazingly better than current iterations graphically as the difference between WarCraft 1 and even StarCraft.
