By the time I bought D3, with the release of the expansion (and armed with the knowledge that I should do so by prior experience with D2 LOD and even D1 Hellfire), it had actually been made into a half decent game. The true endgame kind of sucks, but it sucked in D2 as well, and up to that point I got a good 200ish hours out of it that I didn't regret.
D4 is just not good though. It's a skeleton of a game and the bones are stale. Diablo and Overwatch were the only things I was still holding out any hope for Blizzard for and they pretty much ruined both those franchises. So I won't be buying Actiblizz products anymore. They've made their point.
Potentially. But probably not at this point. They'd have to do such a huge about-face as to be a completely different game.