From Diablo wiki: “The roots of Diablo IV lie in late 2013 or early 2014, where was cancelled, with the instruction that Team 3 move onto Diablo IV and Diablo Immortal, regardless of what form the game might take. Team 3 was split up, with some of its members transferring to different projects within Blizzard. Those who remained were assigned to provide patch support for Diablo III while simultaneously developing Diablo IV. Development of the game began in 2014. The initial project was referred to internally as “Hades”. However, Hades was cancelled in 2016, and Team 3 began work on Project Fenris.”
It took them 9 years to develop Diablo 4. Internal problems in company consumed a lot of this time, but still, i believe 5 years from now is the most optimistic (or pesimistic, i am pretty sure they are not capable to make a good “Diablo-like” game) scenario for making Diablo 5.
Don’t want to loose cosmetics? Don’t buy them. It’s just a fact that you are affraid of that means you don’t trust Blizzard. Me neither. It’s already a scam that we have to pay for our chars not look like dogshit in a full priced game.