The latest iteration of Blizzard's Diablo franchise is a divisive title, even years after its initial release. It had an immensely successful launch, followed by wide criticism of its various problems. The player base then dropped as those who were expecting it to be the second coming of Diablo II found it rather different and unsatisfactory. However, fast forward to a few years later, it is still being played by a sizable community, so it is certainly not a bad game.
Nowadays, Diablo III has taken a form that is far from how it was back in the early 2000s, and yet is still faithful to the spirit of the first two Diablo games. Diablo III now occupies the same space that other games like Warframe and Path of Exile are in, and they have a lot of similarities in how they handle their content as well. But that's not that hard to surmise since those other games took inspiration from the Diablo franchise in the first place.
Before, it was still mostly a singleplayer experience. There may be online multiplayer through Battle.net or local multiplayer through LAN, but most players during the heydays of Diablo II played on their own. Later on, the online multiplayer scene did expand as broadband Internet became more prevalent, and it breathed new life to the game. But Diablo III was designed as an online experience to begin with, albeit not like an MMO.
Blizzard has adopted the seasonal structure to Diablo III, which gave players something to watch out for and more motivation to keep playing the game. Many gamers are into the grinding gameplay of action role-playing games, and Diablo III is an improvement of the original. However, it wasn't before it had been momentarily superseded by games that were inspired from Diablo.
It was like a give-and-take sort of deal in that Blizzard then had to learn from those that learned from them earlier on in order to bring Diablo III to the state it is in now. Perhaps you can say the game is now in a place that's more niche, but that actually works to its favor. While more mainstream audiences have written off Diablo III as a "failure" of sorts, those who are enjoying the game need not concern themselves with that since they are seeing how it is not a failure in first-hand.
There were plenty of hurdles that needed to be overcome during the time period between launch and now. The Auction House being removed was a big one since its absence changed the game for a lot of people since there was effectively little to no more trading between players in the game at that point and pretty much all Diablo 3 items are looted. If you think about it, removing most of trade was a brave decision that many other developers would have never taken.
Those who did stick around in the game saw regular improvements and a good number of them are still on board right now. If there ever is a reason for a gamer to get into Diablo III, it's that if they like action role-playing games and similar titles, then Diablo is pretty much at the top of that mountain, even after all these years.