If you don't see a difference in how progression works in D2 (pure arpg) and D4 (MMO-like arpg), there's no point arguing.
No, I don't see any. You slaughter foes at high speed and earn XP and loot. That's it. I don't even see what kind of relevant intricacies I could be missing here, sorry.
Yes, D2 didn't have endgame in modern terms, but everything up to that point felt great.
D4 feels even better than D2, at any stage of the game, because it is technically far superior than D2, as in it clearly profits from a quarter century of experience. Even D3 forgettable as it was feels better than D2.
In D4, you do campaign once, then on subsequent playthroughs look for ways to speedrun the leveling process
Wrong. In D4 you press the button 'skip campaign' to not having to endure the campaign again. Which is only possible b/c D4 is "open world", which is a good addition to the series. I still remember wishing for an open world (and a 'skip campaign' button) in D2.
the highly coveted endgame, which puts activities ahead of itemization
And I presume it shouldn't? What would 'putting itemization ahead of activities' look like and why would it be better?
Even with the current "loot reborn" finding something interesting feels like tedium.
I run a dungeon or two, knock over some loot goblins and I have to the vendor because I am full and have to sift through the mountain of loot. What exactly here is 'tedium' beyond of what is part-and-parcel of the genre?
And even with the so-called variety of endgame activities in D4, you're still railroaded
You're not railroaded if you have got a choice. In D2 you have got no choice because there simply isn't anything to do.
into doing the most optimal ones that yield the best items.
And that one choice is better than another at a given point in time is an unavoidable consequence of living in a universe with entropy. What is Blizzard supposed to do about that? Play the game for you? Simply don't do the most optimal one if you don't like it and do another.
In D2, you make your own endgame, there is a moment where you can say "I'm done with this character and want to start a new one."
What prevents you from doing the same in D4?
they want to make you chasing that carrot forever, as bloated as it is
No, you (collectively)
want a carrot to chase, Blizzard just delivers that. Chasing the
carrot goblin is the point of the game. How can Blizzard even win here? Not put any content into the game whatsoever besides the campaign, aka D2? Many people will complain, including you, for lack of endgame content.