It's common practice in game industry, each successful breakthrough game spawns clones of various quality. Doom spawned tons of first-person shooters, WarCraft spawned tons of real-time strategies etc.
Heck, even your example of Warfcraft wouldn't have happened because by the pro-IP crowd's standards, it would have been a "rip-off" of Dune II.
I remember when the tech community fought overly broad IP claims and patents, now its just pro-IP shills left and right. Its incredible how far we've fallen.
Dune 2 -> WarCraft -> WarCraft 3 -> DOTA -> League of Legends.
Come to think of it, the only time Blizzard invented the whole genre was with Diablo. All their other (incredibly successful) games were refinements of prior work by other game developers.