In the calculation I saw, the old quests gave more gold, but playing time gives more in the new system. Not saying it's the one true calculation.
The old 10g/3win was a puny bonus relative to quest gold. Quests are not worth a lot in the new system, while game time has become more important, making the system super grindy as you mention.
So just adding that I think your interpretation of the mechanics needs a little adjustment. The new system is btw ONLY better if you just play shitty meme decks for ages or maybe if you only play BG. Any ranked player will lose gold.
I think activision has added a lot of greedy investors to the world of Blizzard sadly. That they don't even try to make their product look more buyable is even sadder
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