Good thing you don't understand how an American publicly traded corporation works at all.
When you're a publicly traded company in America, you are a slave to your shareholders (sad but true fact. There are pros and cons to this). And guess what, shareholders want their shares to make them money. If their customers are happy as a consequence, that's nice. But the bottom line decisions are made with shareholders as the top priority (remember the whole thing where everyone got pissed when it felt like Diablo was neutered to suit consoles? Yeah, bigger market = bigger money).
Blizzard realizes they need more stable revenue streams, this is why they have Hearthstone and Heroes of the Storm. Right now, ignoring the Call of Duty Publishing Simulator 20xx side of Activision-Blizzard, WoW is by far Blizz's biggest and most consistent cash cow.
Okay, let's frame this in a way you'll understand (happy cake day btw). Let's say you're the Lord of Terror, Diablo. In order to measure your success as a Lord of Terror, you need to conquer lands. The more lands you conquer, the happier your denizens of Hell would be. If they're not happy, your sack of shit brother Baal takes over. So over the years, you spend grueling hours embarking your generals and soldiers (IP, projects, man hours) to various lands to conquer. And boy do you conquer them, you conquer lands rich in minerals and other shiny things (think of these as past successes, like Starcraft/Warcraft/Diablo). And minerals and shiny things are good for business! Very high value, and it comes in quantity.
However, you soon notice that, once your armies mine all the things from this land, it doesn't grow back and now the lands possess substantially less value than before (diminishing returns, especially over time, especially in the game industry). Well you can't just do nothing, you have to keep conquering lands, otherwise you can't pay to maintain your legion and your standing as the Lord of Hell.
But then, one day in one land you conquer, you notice that instead of abundant minerals, you find land that has rich soil. Instead of having something that's extracted once, you can use this to periodically farm resources providing a constant and long term source of revenue for your reign of terror. Well naturally you want to keep the way of things still so you're still out looking for lands with minerals (Diablo 3, expacs, etc), but at the same time, you're looking for lands with this new rich soil stuff (HotS, HS). You're already the master of mining minerals, you've been doing that shit since forever, but investing in getting the most out of your farmland? That's new, that's innovative. You need capital for innovative.
So then one day you get a bunch of people coming in complaining (that's everyone anywhere that complains about lack of content and upkeep in Diablo 3). They tell you what they want, they want you to take more resources from your farmland and divert them to these old ass barren mineral lands. And you say to them "I already maintain the old lands well. Even the oldest land (Diablo 1) is still habitable." But that's not enough for them (read: you). So you say, fine, make your case. If I invest resources in our old lands, what are my returns. And of course, there are no returns so you say "None." And then Diablo explains that he can't gimp the returns on farmland for no reason and lose out on future gains.
tl;dr: Wait for Diablo 4 if you want more content.