Blizzard President Mike Ybarra stated on Twitter that Diablo 5 won’t spend a decade in development as the previous Diablo titles did.
Diablo 4 is the fastest-selling Blizzard game of all time, so it makes perfect sense for Blizzard to allocate more resources to the development of future Diablo titles, which can result in less wait time between the games.
Perhaps the acquisition of Activision Blizzard by Microsoft will also have a positive impact on the development and may speed things up a notch.