Diablo is not dead yet. The reason we do not get enough information about exspansion / Diablo 4 is normal. They usually come with statement when they know and are sure of what will come.
Diablo franchise still has a large sales potential and fans. But the Brand still suffer huge image/brand problems due to all the problems since release.
The developers are at a crossroads.
1. Increased competition from similar upcoming games Lost Ark etc. Making Blizzard withhold information for their upcoming contents and features. But maybe we get some surprise news at BlizzCon.
2. Diablo 3 is a Conflicting product with too much different views philosophy. The biggest mistake seen with hindsight is that Consol should not have been implimentet. Because the sales was decent, but limited the PC versions ability to be more "explosive" in every form.
AH could have been managed. They have spent too much time solving problems rather than developing content. If Diablo 3 was smooth from day one, we might had have enough content to keep us entertaint and keep our loyality to their products for "free".
So we will see Diablo in the future. Sales history speaks for itself. They must "just" get their act together and develop a unresistable product with super content. I have no doubt that we will see something good. Because it cannot get any more bad then it is?