Goray.... not so fast....the bumper is sacrificial to protect lights, radiator,,suspension etc. The bumper has to protect those things not itself at very slow speeds <= 8km/h
BUT you cannot do that by making the bumper out of a piece of railroad track either (sadly) because
the bumper has to do its bit to collapse at just the right rate in a faster collison to ease the loads on the rest of the vehicle so that the combination of: passenger compartment shell, door latches, A pillars, seats, seat mounts, safety belts, airbags, hood latches, engine mounts can keep you alive at 60km/h or hopefully quite a bit more.
I guess that back bumper in conjunction with the spare wheel where there is one has to protect tha back lights and suspension pickup points AND importantly, the gas tank integrity! Of course that has to collapse at a controlled rate as well unliess you want a broken neck.