All they honestly had to do was update the graphics from D2, expand on the story from the previous, and add new loot items and it would have been a grand slam. endless bitching and moaning about how Blizzard never does anything new, never takes risks, the gameplay is so dated, it's just click click click click, the original is still better, blah blah blah blah blah if I wanted to play a clone of Diablo II I would've just played Diablo II.
Fixed a minor error in your post.
One thing the gaming community has taught me - they are, literally, the unpleasable fanbase. They will whine about everything. Take Road A, whine that you took Road A and not Road B. Take Road B, whine that you took Road B and not Road A.
The D2 dev is feeding into this, proving he's quite the opportunist.
So no, I don't have sympathy for him. What he did was pretty low, and the D3 devs are right to be angry with him.
Also, you can't discount the version that was scrapped. That was still time spent on the game. Those man-hours didn't magically disappear because the work was tossed in a bin - those design choices still had an impact on the final version.