The problem, as I see it, is that the focus is always on tweaking the "endgame." I just find this to be silly. Once you get to the endgame it's pointless, and people basically blaze through the base game on purpose. And once you get there it really is no more than a slot machine that caters to people with serious gambling addictions. You run the same dungeons over and over again, sometimes thousands or even tens of thousands of times, all for an item with slightly different numbers. In Diablo 1 and 2 the item often doesn't even look different.
But I've always found the base game to be excellent. Fun, exciting, and a blast with 4 people.
I feel like, particularly with the new Diablo 4 they could make the leveling curve on the base game last MUCH, MUCH longer and it would actually be far more enjoyable for far more people. It would piss off the endgame crowd though, because all they want to do is get to max level as fast as possible so they can start farming.