" Why doesn’t Toa Nui exist? It is theoretically possible, isn’t it?"
Greg answers:
“No, it’s not. Toa Nui is a myth. Can’t exist. Toa bodies, as they currently exist, would burn out if carrying all six elements.”
Personally I am kinda disapointed. I always tought when push really came our original six could join up and defeat final big bad, what ever it was.