Here's another way to look at it... maybe it might open your mind a bit.
You spend hours coming up with original content for a mod, new characters, story etc., you put it all together, and pump it out as fast as you can to the public.
Then someone comes along and likes what you have made, but then he remakes your work adding much better Graphics, 3D models, removing bugs, making it less clunky, suddenly his mod is getting more downloads.
Sometime down the line this guy's got way more popular than your mod, he's getting donations, praises, and even getting an offer from the original developer.
I think it's quite clear why people just don't want others to copy their work and ask people to be at least original enough to actually earn their rewards.
Your comparison sucks. You're comparing someone making a mod from a highly successful game that's already made the company lots of money, to someone copying a mod from another mod. Bad comparison is bad on so many different levels.
All this guy wants to do is remake a game he loved so much into a Divinity mod, and share it with people who might also enjoy it.
I mean, as long as he's not trying to create a whole new game with the Diablo name and trying to sell it to the general public, I really doubt it's ever going to be an issue.
Again, other people have made Diablo mods in other games and guess what? They are still around. Which means what? Blizzard didn't care enough to do anything about it.
So since this argument is going in a circle, and since you can't do anything about what the mod creator does with his time, and since I really don't care about what you think anymore, I'm going to stop replying to a useless argument.
Have a nice day!
