I won't criticize Diablo itself, because I'm ignorant of it, but let's talk about Diablo-clones.
Diablo clones are extremely similar to a certain kind of traditional RPG: bad single character roguelike type RPGs, where you control one character, grind stats and meaningless gear upgrades, explore random dungeons and fight monsters by bumping into them. (This kind of game can be done well occasionally, but there are hundreds of forgotten and terrible variations of this formula.)
That's a degenerate form of RPG, but it's still a very recognizable form of the genre, that may actually be more fundamental to the genre than the actual good games that came later. Diablo clones are basically the same thing, plus dumb clicky combat.
Similarly, for a while "action RPGs" used to just be JRPGs with awful action combat replacing the awful turn based combat. Usually bad games, but almost the exact same thing as their turn based counterparts, just with more pointless button mashing.
Classifying these games as RPGs wasn't much of a stretch.
On the other hand, we have games like Deus Ex or Dark Souls, which may actually be good, but that feature gameplay which is fundamentally identical to a stealth shooter or a third person action game, just with the addition of more stats, more dialog and more story.
If Deus Ex and Dark Souls are RPGs, then so are those sports games, where you create a character, grind his stats, and play through a story mode.
