This is not accurate. You are confounding the loot system with loot drop rates. In early D3 vanilla, the drop rates were very low. Currently in D3:RoS, the drop rates are very high. Throughout D3, it always has been personal loot. This clearly disprove the concept that personal loot is associated with high drop rates.
This is what people are asking for (with automatically being RNG/binomial probability based). The total drops between personal loot and FFA loot being the same.
D3 calculates (in general) for each character per monster kill; however, this could be calculated globally and then “assigned” afterwards.
This is not the right way to think about it as described above. Borderlands 3 and PoE have optional loot systems. You can see for yourselves how these game implemented them. For PoE, there is FFA, timed loot, and personal loot where the game creator sets upon game creation.
Except it doesn’t change the core. Plus it would be optional anyway. So if you thought it changed the core, you could choose to play the old way. I would encourage you to change your title, as it appears to me that you do not understand what personal loot is.