Thermodynamcis favors O2 and H2 forming water but this doesn't happen appreciably at room temperature. Proteins would not break down easily in a world devoid of life without enzymes to metabolize it. The amide bond is strong and does not break down easily without catalysis at neutral pH. Many polymers are stable compared to other biopolymers (rubber, ER, PP). Biopolymers tend to be more reversible because their biological role demands it. I'm not disagreeing with your overall point, but your statement is a bit of an overgeneralization. In a world without life to digest them many polymers would be more stable eventhough they would need energy to form.

Regards,
Mark