What you’re asking is a power multiplier for the amount of money you spend on sub.
Multiboxing isn’t about that, You have to gear all 5 characters individually. They don’t all do the exact same DPS. There is a difficulty and skill learning curve to be efficient with multiboxing. 5 human head will always be stronger than 1 human head.
You are trying to make it about the player when its only about the character power.
Your proposition is a flat damage increase x5. No penalty 100% character control all across the board. No losing 1 of your character to CC, off-sync or obstacles. You would ALWAYS have your permanent 5x and perfect mobility.