The whole idea reminds me of the springs under the seat on my old John Deere garden tractor, which had two huge 2" U shaped springs on which the seat pan attached.
They looked like two truck springs bent in a U shape. They would last a couple of years then either sag or snap at the bend. After replacing the third set I found a couple of huge valve springs from an old engine to add support to rear end of each spring and they lasted for another 10 years.

I could picture a set up like that on a bike could lead to a pretty nasty injury when it fails, and find it really hard to believe that a small round piece of steel like that would have any longevity as a spring that could be supporting a couple of hundred pounds. The likely hood of metal fatigue early on would be pretty high.

In a coil spring, the stresses are spread over the entire coil and each coil moves very little, on that seat spring, all the flex is concentrated in one area and movement is concentrated in one area.and that's bound to fatigue and fail.