Robust Mirror

You’re not wrong. They’re designed to burn up completely but there have already been failures and documented cases of 2.5kg pieces hitting the ground. The FAA predicts at current trajectories we’re looking at about 1 person hit every 2 years by stray debris. And it’s only going to get worse the more they launch.

They’d last as debris for about 5 years before falling. Atmospheric drag among other things causes orbital decay that cause them to eventually fall to earth without adjustments.