“This tells me is that at least half the player base quit by act 3, yet this game is supposed to be some kind of magnum opus”

You’d be surprised to find out how many people don’t actually finish the games they play. It’s actually pretty common, and not a good metric to judge a games quality. Just go look at trophy/achievement data for other popular games. Its like this across the board.

For example: only 45% of players have the trophy for beating the final boss in god of war Ragnarok (one of the best reviewed games from last year, and PlayStations fastest selling exclusive ever).