Congrats, you understand what a zero sum game is. Guess what? D:OS 2 is not a zero sum game - at least, not until probably the very end of the entire game.
It is not possible to travel between acts without your full party being with you. Can't be done. You can't race to the final boss and leave your partner still on Fort Joy. Every time you change maps your partner has to come along.
Yes, there probably will be some combats where you'll want your party member hindered for. But there will be quite a few combats where you'll want - you'll NEED assistance to not die. A crippled and useless party member won't help you progress in your own goals. I see this clearly with Ifan, who is basically dead weight in a fight with his crappy starter daggers.
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All-in-all, the solution to "jerk co-op partners steal all my loot" is "find better co-op partners", and as a plus that's easier than adding instanced loot which the game is absolutely not designed for at all. It's drop in, drop out co-op, remember?
You can't open a container twice and get two different things. What happens when the co-op partner leaves? The host has access to both the contents of the single container. Instanced loot would literally be giving co-op players double the loot of single-players.
