I think the issue with D3 is that Blizzard is no longer invested in titles that cannot give them returns. At this point, they are probably well aware that they can sell expansions with the promise of fixing all your problems and making your dreams come true, even if the actual content is shallow. Their marketing team is that good and their fans are still that loyal. So, why put out effort on a game that doesn't have a sub or a cash shop? They tried the RMAH and it sucked, and as surprised as I am that they removed it, they have no reason to continue to support the game now.
Let's not forget that they are the stepchild of the famous Kotick now, a man who has made a legacy out of remaking games with very little changes to re-sell to fans over and over again. He has explicitly stated he has no interest in titles that cannot be manipulated for long term gains, and his ideology has very obviously seeped into the management of Blizzard, in my opinion. D3 gets minimal effort because it gives Blizzard minimal profits, that just makes business sense and Blizzard's job really is not to make a great game, it's to make a great profit. While we'd like to think those go hand in hand, minimizing cost is part of the equation as well.
Call me skeptical, but you can look at the other titles in the genre and see how popular it is, overall. PoE has a fair amount of players but still boasts player records that are well under 10% of the supposed Diablo player base, and I would assume D3 isn't any better after this long. I would honestly love to pay a sub for D3 and see it become half as good as D2 was, but I just don't think this genre is big enough for anyone to care. Despite how good they make the game, I think Blizzard knows their fans will get bored of it in two months anyway and go back to WoW or their other 20 little games they're spitting out.