One of the more important skills of good game design is to understand that whenever your players are complaining about something, there is something wrong that you need to identify and address whilst also recognising that it's rarely the thing the players think is what's wrong (as they just see the negative end result) and that they tend to express those complaints as demands for the solution they think is best to what they think the problem is.

In this case players are yelling at Blizzard "There's not enough content!" when in fact, as you've observed, there actually is plenty of content, it's just (seemingly, I've not actually played it myself to say for sure first hand) that Blizzard made it too easy to optimise your way past all of that content as a minor inconvenience on your way to, uh, nothing.

The answer to the problem is twofold. One you need to plug those holes in your balance so players are no longer incentivised to optimise their way past actually playing and enjoying your game (now I talk about it I think I vaguely remember reading an article that Blizzard are doing exactly that and having a hard time cleanly pitching the benefits of it to the player-base, which is why you also need to.) Two, try to put the horse back into the stable by now, sadly, actually having to create the end game content that players have bursted their way through to because your game design unintentionally promised it would be there (or just write those players off as a lost cause. Which seems like a dreadful idea as they are the ones who were the most passionate early buyers of your product...)

Alternatively... If they'd caught these issues before release (which is often, though not always, a matter of giving the developers and designers the resources to do so) they could simply have caught those issues of optimal builds being too powerful for the content and adjusted either or both to be a better match and ended up with a title that players liked more than they will like the harder to make version Blizzard now needs to turn Diablo 4 into (not to mention, that the work they need to do to introduce worthwhile end-game content could have just gone to a paid expansion for their more well regarded release instead.)

But then the Bobby Kotick's of the world are boastfully proud of their complete inability/unwillingness to think about the development of their games in that way so here we are...