Checked expiration dates, they were comparable to store-bought. I store my cereal in pretty abhorrent conditions sometimes (like leaving it in the back of my car during Texas summer for a few days) and never noticed degradation leading to flavor/texure like this, but storage issues are still a possible cause. Maybe I'll do a real experiment sometime involving storing grocery cereal outdoors in the Houston summer and see what happens.

I completely agree with all your sentiments and part of why it took me so long to come to my conclusion.