"Old China ever had polygamy system, that's how our movies/series still have so many materials to show how the wivies plot against others to death."

Well its not that old! My wife is Chinese and her grandfather had two wifes. What I can say is that is does necessarily complicate things are there can be anamosity between the half brothers and sisters. Also i t can create problems for inheritance as it "dilutes" the estate of the father/husband - assuming that he has mulitple children by his different wifes.

Also - re the China example - I understand it was usually only the very rich that would take more then one wife.

My question is - as put to me by my wife when I jokingly raised the idea - "well you can have 4 wifes if I can have 4 husbands". Lol - so why not then? Why can a wife not take 4 husbands?