I feel like in order to continue the series they really didn't have a choice BUT make a clean break from 3. Malthael wipes out an enormous portion of humanity. Like extinction level event numbers. In order to have a functional game world, there's just no way to sew these two issues together logically - addressing it would result in basically every town being completely empty. In comparrison, in real life the Black Death "only" wiped out a about a third of Europe's population, and that resulted in entire towns and villages being depopulated and abandoned.
Is it weak sauce they don't acknowledge this even a little bit? Yes, yes it is, but I get why they did it. I'm hoping they eventually explain it in universe, even if it's something as simple as Imperius/Belial/someone else basically pulled a Dr. Strange in No Way Home global memory wipe.
Alternatively, they should have made the game take place MUCH further in the future, which would also make more sense for the population numbers.
Likewise with the heroes of 3 being more or less sidestepped. The whole Nephalim thing was basically starting to scale up to Super Sayin levels of power, so it makes sense to sort of "reset" the power levels. Again, I do wish they had addressed it, because the ending of RoS more or less implied that becoming a potential issue (and maybe that's where Tyrael fucks off too in D4), but I get why they had to scale things back.
But Diablo as a series has always been absurdly bleak, 3 was the only one to break from that mold, which is why many long term fans mocked it's cartoonishness (both in writting and in aesthetics). The Angels have always been just as shitty as the Demons, they're just less explicit about it. Only Tyrael, Aueriel and Itherael see humans as anything more than a mistake/abomination (Out of the Angiris Council anyways. Some of the lesser angels do too), and Ty's the only one that actually gives enough of a shit to actually try to DO anything for the mortals.
I definitely do feel like the long term endgame probably IS an assault on the High Heavens and Burning Hells both and ending the Eternal Conflict by destroying both sides for good - which, honestly, is where I suspect the cancelled D3 expo and/or earlier version of D4 we never got was heading.
D4 (actual release version), I enjoyed the plotline, but there's VERY obviously points you can see where the game's years in development hell (pun intended) and total turnover of management resulted in some... parts that don't quite fit seamlessly together.
But I can get why that's tiring - I personally just fell in love with the look and lore of the series way back when and to me the bleakness just works for the endless horror vibe, but I get why that doesn't work for everyone. That said, to the best of my knowledge, the only heroes that were corrupted are the 3 from the original game: The Dark Wanderer is Aiden (still hate that retcon), Leoric's son, the Warrior, who becomes Diablo's host body in 2. The Rogue is Blood Raven, a boss in D2's Act 1. Likewise, the Sorcerer becomes The Summoner, who you kill in act 2 to find Tal Rasha's tomb.
For D2, the only hero class that we know was killed is the Sorceress. Canonically named Isendra, she's actually the Wizard's mentor, and you meet her as a ghost in RoS if you play as that class. She was murdered by an Assassin, who may or may not be Natalya, the prototype of the class you encounter in Kurast in act 3 where she mentions they kill magic users who grow too powerful for their own good. You never see Nat again and the player class wasn't introduced until the LoD expansion, but it's clear they're one and the same. Natalya has a named Demon Hunter Set in D3 (Natalya's Vengence), the flavor text of which implies she may have helped create the Demon Hunters and may or may not have some past regrets, which seems to imply she did indeed kill Isendra and then regretted that decision. It's also not a stretch to assume Nat *IS* the canonical Assassin as both it and the Druid don't join the storyline until LoD.
The D3 (male) Barbarian was originally to BE the D2 Barbarian before they abandoned the idea because it was too difficult to make work storywise. That's why he looks so damn old though. The Cannonical Barb vanishes in the aftermath of the Worldstone's shattering, likely broken by the destruction of Mt Arreat. My headcanon is he's the founder of the cannibal tribes you see in the Dry Steppes in 4, because that idea just fits the universe to me and it'd track as the Crane tribe are the descendants of Nihlathak's kin, exiled after his treason.
The Necromancer is Xul. He survives and is as normal as a Necromancer gets. You meet Mehtan, his apprentice in act 2 of D3, where he confirms Xul once defeated Diablo himself. Nothing further is mentioned, which leads me to believe Xul ended up either dying normally, or is still around somewhere, possibly corrupted by 4. He's a Necro, the rules are different for them.
Cassia, the Amazon, survived and became Skovo's military commander. She's alive after the culling of humanity in RoS, as Lorath mentions her, though not by name.
There's absolutely no info about the Druid. He apparently just fucked back off into the woods.
Doubly weird since the Druids play such a big part of Act 2 in D4. Suspect he's gonna show up at least in lore in a Season.
Likewise, they never say anything about the Paladin. However, in D4 there's a Paladin named Carthas who apparently helped defeat Mephisto, and he's corrupted by the lingering energies of said Prime Evil during the Zakarum's attempt to entomb Sankekur after you stab/magic Mephy out of him. BUT the only Paladins you see in Kurast that aren't already corrupted are the player Paladins, which makes me wonder if he's supposed to be the canon Paladin and they're implying that he leaves the heroes after act III and doesn't continue to finish the events of the game?
The D4 heroes defeat Carthas's corrupted spirit in that quest, so if he is supposed to be the canon Paladin, he does indeed also get turned into a boss.
The D3 heroes aren't mentioned at all in 4 that I came across. Lorath says that Tyrael left the Horadrim vault "scared", which makes me think that is, in fact, a plot hook implying his last words in D3 about the Nephalim choosing evil came true, and they're going to be bosses in upcoming Seasons or an expansion.
All that said, yeah, if the gameplay loop isn't a big hook, the story doesn't matter, because lets face it, in a loot game like that you only pay attention to the story a couple times at most anyways, and then it just gets in the way