Diablo business model isn't about maintaining active player base because there are no microtransactions / monthly subscription fees, therefore the main goal for the company is to sell as many copies of the game during a release phase, as it is only possible. Note that this is a gaming company no longer, unlike for example Grinding Gear Games, Activision Blizzard is a corporation who has to satisfy its shareholders.
So from the management point of view, it's reasonable to create another expansion for a half-dead game, spend few $mil on advertisement and marketing hype and with a very limited budget steal another 15 * 50 = 750 mil dollars from the naive nostalgic players.
Development of Diablo 4 is very likely to be a reality, however, it will take them a couple of years, and it will surely be no longer a game which will follow a standard business model to which majority of gamers from the 90s and early 2000s are so used to - it will be either based on a monthly subscription or microtransactions (or both), and will, of course, focus on maintaining a healthy, active player base in order to generate a steady income (like other Blizzard games: WoW, HS, HotS, Overwatch).
I work as a management consultant and this is a common practice in the business to squeeze as much money from the devoted customers as it is only possible, as I said do not treat Blizzard as a Gaming company for gamers, it's an enterprise listed on the stock exchange and their only purpose is to make more money.