For a 17-year-old game, Diablo 2 still holds up in a lot of ways. For one thing, it is still a whole lot of fun to play, especially for action role-playing game fans who are into the type of gameplay that the Diablo franchise set the blueprint for. However, 17 years is a very long time for a game to stay "alive," and it has indeed shown its age in a lot of ways. With Blizzard having turned its attention to other newer things, it has left Diablo 2 to languish while it is still being played enthusiastically by fans worldwide.
With that said, those fans have had to put up with a lot of things that have lingered throughout the whole time. There are plenty of quality-of-life improvements that can be implemented in the game, as well as improvements to the infrastructure to make the game run more like a modern online RPG. While Diablo 2 has indeed stood the test of time, it can still use a bit of a boost to help it live on for many more years to come.
The one improvement most people have been wanting for all these years is the merging of the US East and US West servers. The two regions were split during the early 2000s due to limitations in Internet connectivity that time, but it is now a whole lot better. Therefore, there would not be high latency between that great distance. If people halfway around the world can play on a server with minimal ping, which is indeed the norm today, then this improvement should not be a problem at all.
Then there's the obvious improvement, which is the removal of bots, hacks, and dupes. The first two are the usual scourges of online role-playing games, and they have never been addressed in Diablo 2 due to the lack of ongoing support for the old game. As for duping, it is a dusty remnant of the old network infrastructure that Diablo 2 still runs with to this day, like a steam locomotive in the age of the bullet train.
Legitimacy is Diablo 2's biggest problem at the moment, with bots running rampant and hacks being used aplenty. However, it's also a good reason why the game is still somewhat alive today. Players find enough reason to put in time and energy into botting and duping Diablo 2 items, and there is a whole load of things that these practices support in the current Diablo 2 ecosystem. For them to be removed, the game has to start attracting more players to get on board.
The answer to that is the release of Diablo 2 Remastered, and yet it has been stated by Blizzard North to be a task so gargantuan that it's virtually an improbability unless they are willing to commit to it. But if we are to expect Diablo 2 to have its major flaws patched up and its interface enhanced with quality-of-life improvements, then that's pretty much the only thing left on the table.