Dynasty Warriors is built on a good, simple idea: slashing through hundreds of enemy soldiers in a single session. As the PS2 was released, Dynasty Warriors 2 was perfectly poised to take advantage of the technological leap, and we rejoiced in smacking down entire armies.
Dynasty Warriors 3 refined things a bit... and brought in the concept of Xtreme Legends - an add-on pack with more missions. Dynasty Warriors 4 held onto Xtreme Legends and raised it with Empires, a strategic add-on. And so in 2005 we came to Dynasty Warriors 5 and its two sibs. With one idea, somehow Koei had pumped out nine DW games in five years - and two more on PSP.
But it's not just Dynasty Warriors and its billions of fallen Chinese soldiers that's the problem. There's also Samurai Warriors, the exact same thing... but set in feudal Japan, and with its own Xtreme Legends and Empires, of course. There are five of those already.
On the PS3 and 360, Koei's got the upcoming Bladestorm - same thing, but medieval England and France. Ninety-Nine Nights, which had nothing to do with Dynasty Warriors, was considered completely tired just for smelling like it. This concept is not so much run into the ground, as it has founded a giant subterranean civilization of brain-dead computer opponents hitting each other... forever.