If HT bonus room gave rewards to all 12 players for example, people wouldn’t need to multi-client — the profit would be the same using just one character. The bonus room also requires movement and focus to break boxes and loot within a time limit, so no one’s going to loot extra using multiple clients anyway. That means more full parties, faster runs, and more runs per player. Easier to find parties too. If someone really can’t find a party or buff class like SE, they can still mule or multi attack if they want. It’s a win-win. I don’t see any downside or reason it couldn’t work.
While I don't like mulestory, I get it and think it's fine for people to do. I do wish there was more upsides to having unique people form a full party rather than a couple of mules. I doubt the big class rebalancing update will come soon but you know something is wrong when you just need a character to sit in place and use SE, CR, HS, or whatever every few minutes from a safe spot instead of being a participant.
It’s honestly crazy to me that you’d even suggest this. If someone doesn’t enjoy solo content or mule play the solution isn’t to take it away from everyone it’s to find like minded players and do bosses together. That way you get the team play you’re looking for without hurting the players that worked hard to reach those achievements.