How far will Diablo 4 monetization go?

For me it depends on how good the game is. I am likely to spend a bit on cosmetics if the underlying game is good. If it isn't, monetisation is likely to tip the balance towards uninstall.

Play to win feature (like Diablo Immoral) are a red line for me though.
 
Given the recent trajectory Activision Blizzard has been on, I'm basically expecting pay to win at this point. I don't think it will be quite as bad as Diablo Immortal, but still pretty bad. Lootboxes are pretty much a guarantee, and I'd be amazed if they didn't at bare minimum include various potions and scrolls that are slightly better than what can be found in game. Wildly overpriced cosmetic items, for sure, many of which individually cost almost as much as a whole new game.

It's almost like they are chasing after EA. I'm beginning to wonder if we're going to see a Diablo 5 in a couple years, which is literally going to be Diablo 4 with a number "5" sticker on the cover. At this point I wouldn't even be surprised if you had to buy card packs to unlock the character classes.
 
Given the recent trajectory Activision Blizzard has been on

I'm not sure. Diablo Immoral err Immortal is a mobile phone game and pay to win is pretty common in that genre. It's also receiving massive criticism from long-time Diablo fans whose voices can be heard pretty well.

If D4 is pay to win, then I'm not going to buy it. For computer games, I'm paying once and only once, although I could understand a certain subscription fee for bnet play. I would be playing it on single-player anyway and if it comes with a bnet subscription fee and no single-player, I won't buy it.
 
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