When I looked closely at the ones that cracked on me, it cracks along a line where the plastic meets up in the mold in maybe the far end where the plastic was injected into the mold.
I think this was the case, because I can see flow or swirl patterns in the grey coloring of the plastic that shows this. The plastic inside the mold must not have been hot enough when this happens that the plastic material does not fuse molecularly together completely at that line when it meets up in the mold?
Sort of a built in weak point,made during manufacturing, not on purpose, of course....Stress and the off-gassing of chemicals from the part must then conspire to cause it to crack at that weak point after many decades of use.
Just my theories,,,,
I bought those black replacements from American Cyclery, a few years ago in SF, that I think were made for and sold by SOMA and they so far have been working out really good and look very good.