Those plastic bumpers are not bumpers, but bumper covers. The real bumper is under the bumper cover. It is designed to absorb impacts. It is quite substantial. The bumper cover is just to make it look good and does fall away in the event of a collision. You should be able to see the bumper in the Sienna by opening the hood and looking down in front of the radiator.Well you need to remember that 90% of today’s cars are just a bunch of cheap plastic and soft metal. If you’ve ever seen a rear end crash up close- the front of the car just comes flying off. Like a kid crashing a lego vehicle. And that’s why you see so many crashed car bumpers just left laying on side of road . Don’t ever assume that you have a real bumper that could take a minor collision.