Some selective quoting there. If this really is supposed to be a Diablo clone, the replays is why you will play. Finishing D2 the first time was basically a trial run for Nightmare and Hell difficulties, when you'll actually hit the top of your skill trees.
If that's the case, I imagine it will take 3 plays to fill out your skill trees, which is what this kind of game is really about. It's similar for all games I've played in this genre - Diablo, Titan Quest, Sacred, etc.
Whether it will be worth it is another thing, but I don't think you should measure this kind of game on the length of your first run-through only.
The first playthrough is what everybody who picks the game will do, not everyone will go through the game a second time, the first time through has to be quality, regardless of all the phat loot the game offers.