You can blame the parts for being junk from the get go. Rarely do people own up to their own heavy handedness. Regardless, it's quite a stretch to think GM expected the parts to last 20 years without regular wear, tear, weather extremes and time doing what they will inevitably do.
I got fed up with the issue and cut a couple of pieces of stainless sheet metal to size, filed and buffed the edges smooth, drilled holes and the armrests are now more stable than stock. I actually do use them as door pulls too. Done correctly, it's the best option to shore up a part that was never intended to endure what the existing ones have to date.
If it's a stock plastic part, it is FRAGILE and BRITTLE. Blame GM or whatever, but it simply is what it is.