Hum based on your post you do have a point. I've always seen RPG as an conjuction of various sub-genres that made for the whole. Like Feylic said, each sub-genre is way different in it's core.
Zelda is different from Chrono Trigger, Diablo is different from Persona, Final Fantasy is different from Fallout and so on.
But take nowadays gaming standarts. Which genre is trully unique, in a way that it doesn't involve any other mechanic than it's core base in which that genre started? You have FPS's that use a leveling up system much like the RPG genre.
You have the adventure genre that mix more and more platforming and puzzle mechanics that were typical point-and-click genre oriented.
You have racing games that also use a leveling up system or mechanics that we're only found in prior MMORPG games.
Today, a genre is not defined as it was in previous genres. As games evolve, the boundaries between genres will dissolve even further.
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