According to what metrics? D3 shards chat servers and matchmaking. It also isn’t cross-platform, where probably more than half of the players are on console.
D2 has a very devout playerbase of neuroatypical/ASD players that have been obsessing over it for decades now. There is nothing wrong with that, but it’s like a smouldering brown-dwarf star that will glow like a warm piece of iron until the heat-death of the universe aka they shut the servers down. It will never shine again, but it won’t completely fade into oblivion either.
What worked 20 years ago, in your eyes, doesn’t hold up to today’s standards and evolved consumer base. If you like D2, go play D2, but stop trying to force it on D3 and now D4.