They've confirmed they are switching to a yearly expansion model, ala Destiny 2:
Rod Fergusson, General Manager of the Diablo franchise, has confirmed in an interview that Diablo 4 will get annual expansions.
www.wowhead.com
At first, I figured I'd wait til the game got its normal expansion and pick it up on the cheap later on, but this pretty much confirms I will just never play this. They really only care about money at this point. Cosmetic microtransactions, season passes
and yearly paid expansions. smh