To be fair, Diablo 1 and Diablo 2 both kind of ended on that 'To Be Continued' note, too.

Diablo 1's story was centralized around "Kid gets Diablo's soulstone jammed in his forehead, becomes corrupted by it, turns into manifestation of Diablo", and the hero ends the game by jamming Diablo's soulstone into his forehead, with an ominous "I can feel him trying to claw his way up from the dark recesses of my soul" line.

Diablo 2 ended with the revelation that Marius had just given Baal his own soulstone, thinking he was Tyrael. Baal then kills Marius and walks away with it.

Not that Diablo games haven't gone down the shitter, I agree with you there, but calling out D3 for ending on a cliffhanger without acknowledging that they've been doing that since the series' inception isn't quite fair.